Implements old and new ways of setting the mag field driver and stepper.

The choice is done according to the Geant4 version used. If the version is <10.4.3 the old
way it's used (i.e. G4MagInt_Drv), otherwise the new method is used (G4IntegrationDriver).
The stepper used by default is the AtlasRK4 if an Atlas patched Geant4 version is used, otherwise it is the DormandPrinceRK56. For the AtlasRK4 stepper to be used, one should define G4ATLAS variable in the
class MyDetectorConstruction