Geant4/GeantV Magnetic field validation options cleanup.

Geant4/GeantV TestEm3 and TestEm5 applications have been extended with constant magnetic field options. The G4/GV TestEm3 applications (general N-layer simplified sampling calorimeter) have been extended with special observables like layer-by-layer mean charged track length and energy deposit. These has strong dependence on the magnetic field propagation that makes possible the validation of GeantV magnetic field transport implementation (against the corresponding Geant4 results). Special scripts/macros for the magnetic field propagation have been provided and all the default application scripts/macros have been recovered similarly to some default settings. The example application README files have been updated with the up-to-date application options.