Add a new way of creating the stepper to be used for newer Geant4 versions
After discussions with John Apostolakis, @mbandier, @jchapman, implemented a cleaner way of setting the stepper. This was needed because of interface changes in 10.6 and also allows us to use the new interpolation driver which becomes available in 10.6.
Keeping the old 'getStepper' method which would be used in case of an old Geant4 version (10.1.3).
Edited by John Derek Chapman