Merge VecCore into master

The new 'veccore' branch now contains the currrent 'master' branch merged with the latest changes from Sandro's VecCore branch and my branch based on his. I made VecGeom compile independently of VecCore by copying BitSet.h and VariableObject.h to base/, so that both VecGeom and VecCore can evolve independently from this point without causing any merge conflicts. It would be interesting to merge this into master after review so that we can have a common starting point for future feature branches and so that everyone can merge the master branch into their branch to avoid conflicts with VecCore in the future.