WIP: Add module to detect/set VecGeom version with git+CMake

This a port of the version module I added to XRootD. It uses CMake and git to set the version based on the existing tags in the repository. When git archive is used to make a tarball, the VERSION file contains the output of git describe, which for a tag will be v2.0.0-rc1, for example. The only thing that is uncertain for me is the VecGeomConfigVersion.cmake. I need to test that the version with -rc part is ok for CMake and won't cause trouble when other packages are trying to find VecGeom. Cheers,