Suggested-by: Alexis Girault <alexis.girault@kitware.com>

This commit adds missing dependencies to iMSTKConfig file.

Assimp was originally introduced in f859365b (ENH: Added Assimp support
to improve mesh reading)

glm became a mandatory dependency in 54a68c1f (ENH: added decals to the
Vulkan renderer)

FindVRPN: The order in which the libraries were
found was creating an compilation error during
linking on Linux, as well as windows when being
read from an external project.

iMSTKConfig.cmake.in: ${PROJECT_NAME} would not be
set to "iMSTK" from an external project.

External_VRPN: VRPN_BUILD_SERVERS:BOOL was defined
twice in imstk_add_external_project(VRPN)

Allows to use iMSTK as an external library
for other projects.

Documentation here:
https://cmake.org/cmake/help/git-master/manual/cmake-packages.7.html#creating-packages