Improve build system to streamline application integration

Summary

Update external projects to:

• support customizing source/build/install directories of each external projects
• support inclusion from within SlicerIMSTK
• be consistently named (e.g openvr -> OpenVR)
• accept customization specifying iMSTK_<project-name>_GIT_REPOSITORY and iMSTK_<project-name>_GIT_TAG (expect for VTK, PhysX and iMSTKData projects)

Add CMake function imstk_add_project_test for testing building of iMSTKProject against iMSTK build or install tree

Update VegaFEM removing obsolete PThreads external project