agheata/removal v2

The branch removes v2, xsec and their dependencies, examples based on xsec, and also old stuff related to Geant4 + tabulations, etc. The flags USE_REAL_PHYSICS and USE_VECGEOM_NAVIGATOR are not needed anymore, removed also the branching these produced in the code. Fixed all warnings on gcc. Using geant namespace consistently instead of Geant. Introduced geant::units namespace (was geant before) for all constants/units. Renamed classes to drop the Geant prefix, moved few of them in inc/Geant to avoid name clashes. Moving to inc/Geant and using #include <Geant/Header.h> should be done properly next.