VecPhys migration to VecCore

This is a work in progress for the migration of VecPhys to use VecCore.
Please do not merge yet, we are still working on fixing remaining bugs, and updating the build system.
Please do use this merge request to provide feedback and ask questions about VecCore and migration.

Update: Bug in ComptonKleinNishina model has been fixed. It is ok to merge this branch into master now, as both on Mac and on Linux it has been tested to compile. However, GeantV does not compile with Geant4MT, only single threaded Geant4 works for me.