VecGeom using external VecCore (without a CMake ExternalProject_Add())
The changes in this merge request make VecGeom use an external VecCore library. No built-in way for installing VecCore is added, since that has dependencies problem that need to be solved beforehand (see my other merge request for details). This version should work fine where the master works now.
Note: Of course, tests will fail, since VecCore is not installed for Jenkins builds.