generated when necessary libraries are not found

Addresses following issue:
https://github.com/google/googletest/issues/1373

Sometimes the sourceforge server appears to be down, stopping us from
downloading libusb1.0. We now add a second url to fetch the libusb
package from data.kitware.com

https://github.com/assimp/assimp/issues/746
https://stackoverflow.com/questions/38296756/what-is-the-idiomatic-way-in-cmake-to-add-the-fpic-compiler-option

The simulation can now be set to either instantiate:
  - A regular simulation using vtkRenderWindow
  - A VR simulation using vtkOpenVRRenderWindow

To do so, the simulation manager can be set to be a VR simulation
when initialized. The default comportment will be a regular simulation.

Unless option have to be passed using the command line, this is the
recommended approach to pass options. It avoids "command line length issues".

Adding of a step for the configuration of iMSTK with OpenHaptics

Contributor: Alexis Girault <alexis.girault@kitware.com>

Contributor: Alexis Girault <alexis.girault@kitware.com>

This will not mandate the users that checkout the code to set up ssh

Add audio testing examples in sandbox. Add resources to the data folder

A cmake option to enable audio is provided which is not enabled by default.
Only audio module of the SFML is build using the cmake options.

Switch to sjh/vrpn to allow for quick changes to the
Arduino vrpn server/client

This prevents linker errors in Windows from finding multiple
copies of the vrpn base classes.

Co-Authored-By: Alexis Girault <alexis.girault@kitware.com>

Expand the imstk_add_library cmake function by exposing the variables
H_FILES (headers) CPP_FILES (source) and SUBDIR_LIST (build interface
directories - the current source dir is always included).

If those values are not manually set, the previous behavior applies,
where all the files and directories in the root directory are used for
that target.

Used that new mechanism for the Rendering target to prepare for Vulkan
integration.

Download Data from Girder according to data key files contained in iMSTK/Data

This uses ExternalData.cmake (Requires CMake >= 2.8.11 release).

Add TEST: Add ExternalDataTest to verify downloaded files from Girder
* Be sure the data requiered for the test is not there (delete it)
* Add the imstk_add_data(${target} ${data_list}) function in CMakeList.txt
which calls ExternalData_expand_arguments().
* It downloads the data provided by the data_list (relative path from
iMSTK/Data)

Add DOC: Add documentation on ExternalData Management
* How to add data to iMSTK from Grider
* Update data
* Use data

Eigen git tag which was currently used (e81ac4502ebbfde43a1e1761b36853d3ead47c33)
can not be checked out anymore as of Sunday, January 29 2017 for unknown
reasons, causing all builds on the dashboard to fail:

http://my.cdash.org/index.php?project=iMSTK&date=2017-01-29

error:
```
fatal: reference is not a tree: e81ac4502ebbfde43a1e1761b36853d3ead47c33
```

The main bottleneck with real-time VTK rendering so far
appeared to be the normals computation. This commit addresses
that issue by making use of the latest normals computation
filter in VTK: vtkTriangleMeshPointNormals.

While this filter is much (5 to 16 times) faster than
vtkPolyDataNormals, it does not check for consistency in the
cell orientations that could cause inverted normals, which
is why the vtkPolyDataNormals is called once in the surfacemesh
renderdelegate to retrieve consistent cells for the input mesh.

See VTK merge request for more information :
vtk/vtk!2271

PS: That MR requires the latest commits from VTK master, which
does not include work made on texture wrap mode nor on multi
texture attributes yet:
- iMSTK/vtk@62a7ecd8
- iMSTK/vtk@ae373026

define "StdVectorOfxxx" for all eigen matrix structures
used in iMSTK within std::vector.

See http://eigen.tuxfamily.org/dox-devel/group__TopicStlContainers.html

Requires to update SCCD to commit 645508940
which also updates its vec3 structure.