Start moving to new slots for SIm10
This introduces a new slot for Sim10 to build against G4 10.7.3 (and potentially higher) to have a one-to-one comparisons against Sim11 (i.e. Gauss-on-Gsino) that should use the more recent version of Geant4.
Note that 10.7 is needed to fully exploit the new fast simulation G4 hook.
It also cleans up some commented code no longer needed.
Edited by Gloria Corti