Considering that in VegaFEM v4.0, pthreads was replaced with
Intel TBB (see [1]), this commit removes the references to "pthread" and
also removes the corresponding custom CMake module.

[1] http://barbic.usc.edu/vega/download.html

It also fixes error like the following reported when integrating iMSTK in
project (e.g Slicer-based app) already looking up threading libraries
using "FindThreads" module provided by CMake:

   -- Found PThreads: C:/path/to/SlicerCustomApp-build/PThreads-install/include/pthread
    CMake Error at C:/path/to/SlicerCustomApp-build/iMSTK/CMake/Utilities/imstkFind.cmake:214 (add_library):
      add_library cannot create imported target "Threads::Threads" because
      another target with the same name already exists.
    Call Stack (most recent call first):
      C:/path/to/SlicerCustomApp-build/iMSTK/CMake/FindPThreads.cmake:8 (imstk_find_package)
      C:/path/to/SlicerCustomApp-build/VegaFEM-install/lib/cmake/VegaFEM/VegaFEMConfig.cmake:8 (find_package)
      C:/path/to/SlicerCustomApp-build/CTK/CMake/CMakeFindDependencyMacro.cmake:90 (find_package)
      C:/path/to/SlicerCustomApp-build/imstk-build/iMSTKConfig.cmake:144 (find_dependency)
      C:/path/to/SlicerCustomApp/Modules/CLI/MyCLI/CMakeLists.txt:33 (find_package)

List of VegaFEM changes:

$ git shortlog be55ca42b..b10672cf1 --no-merges
Jean-Christophe Fillion-Robin (1):
      COMP: Remove obsolete and unused PThreads requirement

List of changes:

$ git shortlog 5be652501..be55ca42b --no-merges
Jean-Christophe Fillion-Robin (1):
      COMP: Fix gcc 5.x build error in sparseMatrix.cpp related to ambiguous isnan call

This commit does not include any functional changes.

It partially reverts 3730f119 (ENH: Model reduction fixed up, toggleable
in cmake, default off) by switching back to using the existing "VegaFEM-CMake"
project where the master branch has been updated to include all the commit
originally associated with the "build_model_reduction" from "vegafemv4.0"
project.

To avoid inflating the size of the project, the following commits
from the "build_model_reduction" available on "vegafemv4.0" have
been squashed together while "rebasing" the changes:
* ENH: Initial commit of vegaFEMv4.0 code
* REFAC: Remove all the extra files that are not code

This allows to keep the size to ~22MiB instead of the ~126MiB.

This commit adds support for passing options like the following:
* iMSTK_<project-name>_GIT_REPOSITORY
* iMSTK_<project-name>_GIT_TAG

This approach enables:

* Streamline external project maintenance by allowing to develop patches
  directly in the source checkout downloaded by external project.

* Configuring the version of each external projects when integrating
  iMSTK in custom applications (e.g Slicer-based app).

Note that iMSTKData and VTK external projects have not been updated.

After updating the CMake minimum required version to >= 3.15, package
export is disabled by default (see policy CMP0090) and explicitly setting
option CMAKE_EXPORT_NO_PACKAGE_REGISTRY will not be needed.

References:
* https://cmake.org/cmake/help/latest/variable/CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY.html#variable:CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY
* https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#disabling-the-package-registry

See https://github.com/oneapi-src/oneTBB/releases/tag/2019_U9

This commit has two purposes:

* update the version of TBB to latest version providing the
  "task_scheduler_init" API deprecated in newer version.

* remove the iMSTK specific logic to look up iMSTK and rely on
  TBB_DIR and associated TBBConfig file.

* rename the external project from "tbb" to "TBB" to be consistent
  with the current naming convention.

If iMSTK_AUDIO_ENABLED is ON:
* include SFML external project
* define iMSTK_AUDIO_ENABLED macro and require SFML package
* require SFML package in iMSTKConfig

On windows, if option iMSTK_ENABLE_AUDIO is NOT specified,
audio support is enabled  and the following message is displayed:

  -- iMSTK Audio support - ON (default initialization)

On windows, if option iMSTK_ENABLE_AUDIO is specified,
message like the following is displayed:

  -- iMSTK Audio support - OFF (iMSTK_ENABLE_AUDIO is SET)

And on other platform, the audio support is always disabled
and the following message is displated:

  -- iMSTK Audio support - OFF (not supported)

This will facilitate the maintenance of external projects added
to the SlicerIMSTK extension by avoiding the "proj" variable from
bring overriden when recursively including external projects.

This ensures external project like the one in the SlicerIMSTK extension
will have their project variable value restored after the call to
"ExternalProject_Include_Dependencies" returns.

This commit supports settings custom value for <proj>_SOURCE_DIR
and <proj>_BINARY_DIR.

This commit ensures the iMSTKConfig file configured in an iMSTK build
tree includes PThreads and LIBFTD2XX variables.

This commit fixes a regression introduced in (COMP: Add relevant vars to
iMSTKConfig to streamline config against a build-tree),

Note that variable for "regular" project are specified. iMSTK specific
variables likes <proj>_ROOT_DIR and <proj>_LIB_DIR are not set because
the location of the corresponding project is automatically inferred by
Find<proj> CMake module provided by iSMTK.

Add variables OpenVR_INCLUDE_DIR and OpenVR_LIBRARY to the CACHE to
workaround issue related to use of mark_as_advanced command in
FindOpenVR and described in CMP0102.

Indeed, this is required because CMP0102 is not automatically set to
NEW and addressing this issue would required to update the FindOpenVR
modules shipped in the different VTK version iMSTK can be built against.

See https://cmake.org/cmake/help/latest/policy/CMP0102.html

This commit fixes the following error reported when attempting to
configure a project like iMSTKProject (see [1]) against an iMSTK
build tree:

    CMake Error at /path/to/cmake-3.21.4-linux-x86_64/share/cmake-3.21/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
    Could NOT find OpenVR (missing: OpenVR_LIBRARY OpenVR_INCLUDE_DIR)
  Call Stack (most recent call first):
    /path/to/cmake-3.21.4-linux-x86_64/share/cmake-3.21/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
    /path/to/iMSTK-build/install/lib/cmake/vtk-9.0/FindOpenVR.cmake:45 (find_package_handle_standard_args)
    /path/to/iMSTK-build/install/lib/cmake/vtk-9.0/VTK-vtk-module-find-packages.cmake:162 (find_package)
    /path/to/iMSTK-build/install/lib/cmake/vtk-9.0/vtk-config.cmake:131 (include)
    /path/to/cmake-3.21.4-linux-x86_64/share/cmake-3.21/Modules/CMakeFindDependencyMacro.cmake:47 (find_package)
    /path/to/iMSTK-build/Innerbuild/iMSTKConfig.cmake:150 (find_dependency)
    CMakeLists.txt:8 (find_package)

[1] https://gitlab.kitware.com/iMSTK/imstkexternalprojecttemplate/-/tree/master/iMSTKProject

This commit fixes the following error:

  $ ctest -R CodeFormatTest -V
  [...]
  test 1
      Start 1: CodeFormatTest

  1: Test command: /home/jcfr/Projects/iMSTK-build/install/bin/uncrustify "-c" "/home/jcfr/Projects/iMSTK/CMake/Utilities/../../Utilities/Uncrustify/iMSTKUncrustify.cfg" "-F" "/home/jcfr/Projects/iMSTK-build/Innerbuild/Uncrustify.list" "--check"
  1: Test timeout computed to be: 1500
  1: process_source_list: fopen(/home/jcfr/Projects/iMSTK-build/Innerbuild/Uncrustify.list) failed: No such file or directory (2)
  1/1 Test #1: CodeFormatTest ...................***Failed    0.00 sec

This commit fixes a regression introduced in 0ffa63e9 (COMP: Add relevant
vars to iMSTKConfig to streamline config against a build-tree) by ensuring
REQUIRED parameter passed when finding iMSTK is not passed when looking up
vtkRenderingExternal and vtkRenderingOpenVR projects.

Introduce variables Libusb_DLL_DIR, Libusb_DLL_NAME, Libusb_LIB_DIR
and Libusb_LIB_NAME for use in application implementing their own
external  project including the ones provided by iMSTK.

* Use Libusb_PREFIX set in imstk_define_external_dirs and remove Libusb_PREFIX ivar
* Remove explicit setting of BINARY_DIR external project argument. It is
  not relevant since this external project download an archive.

Introduce variables FTD2XX_DLL_DIR, FTD2XX_DLL_NAME, FTD2XX_LIB_DIR
and FTD2XX_LIB_NAME for use in application implementing their own
external project including the ones provided by iMSTK.

It ensures the install subdirectory exists when custom location is
specified.

* Use FTD2XX_PREFIX set in imstk_define_external_dirs and remove FTD2XX_PREFIX ivar
* Remove explicit setting of BINARY_DIR external project argument. It is
  not relevant since this external project download an archive.

This commit ensures the iMSTKConfig file configured in an iMSTK build tree
includes variables like <proj>_DIR, <proj>_ROOT_DIR, <proj>_LIB_DIR to avoid
client project to have to specify these project are internal dependencies
of iMSTK.

This commit also:

* introduces iMSTK_VTK_DEPENDENCIES to avoid duplicating
  the list of required VTK components in iMSTKConfig.

* makes uses find_dependency to properly forward parameter like REQUIRED
  or QUIET.

* ensures variables CMAKE_DEBUG_POSTFIX and CMAKE_MODULE_PATH are restored
  to there original values after attempting to find the relevant dependencies.

This commit updates the project to specify the install commands
responsible to copy file to CMAKE_INSTALL_PREFIX only if *no*
custom install directory was specified.

It updates the logic to use g3log_INSTALL_DIR instead of CMAKE_INSTALL_PREFIX.

It ensures the install subdirectory exists when custom location is
specified.

This allows to integrate iMSTK in a project building the required VTK
module externally by leveraging VTKExternalModule.

See https://github.com/KitwareMedical/VTKExternalModule

Introduce variables OpenVR_DLL_DIR, OpenVR_DLL_NAME, OpenVR_LIB_DIR
and OpenVR_LIB_NAME for use in application implementing their own
external  project including the ones provided by iMSTK.

* Use OpenVR_PREFIX set in imstk_define_external_dirs and remove OPENVR_PREFIX ivar
* Remove explicit setting of BINARY_DIR external project argument. It is
  not relevant since this external project download an archive.

This commit updates the module to conditionally pass the "openvr" include
subdirectory specific to the OpenVR install tree generated by the OpenVR
external project specific to iMSTK.

Configuring iMSTK against an installation of OpenVR specified passing
option OpenVR_ROOT_DIR and OpenVR_INC_DIR is now supported.

This commit renames the project so that its name is consistent with
VTK established convention providing the vtkRenderingOpenVR module.

Increase robustness to support case when the option has not been declared
in application integrating iMSTK and re-using the external projects.

Removes obsolete setting of CMAKE_GENERATOR_* variables in imstk_add_external_project,
indeed these variables are now automatically passed to projects from within
the updated imstkSolveDependencies CMake module.

This commit updates the build system to follow the established CMake
convention.

It also allows the use of iMSTK custom find package in the context of
external project named after the library being integrated and avoid
target name conflicts.

References:

* Double colon in target name means ALIAS or IMPORTED target.

  Source: https://cmake.org/cmake/help/latest/policy/CMP0028.html

* This convention of double-colons gives CMake a hint that the name
  is an IMPORTED target when it is used by downstreams with the
  target_link_libraries() command. This way, CMake can issue a
  diagnostic if the package providing it has not yet been found.

  Source: https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html