updated magnetic field and generator for Geant4 10

Some clean-up inside SIM.

Added alternative stepper for magnetic field G4NystromRK4 - it was developed for ATLAS and shows better CPU performance without compromising of tracking accuracy.

Changed logic in selection of primary particles from HepMC in the Generator:
    1) if a particle has status=2 but has no end vertex defined process it as an ordinary particle with status=1, to avoid lost of long lived exotics;
    2) check on total momentum to be above 40 MeV/c; added new option to cut charged particles with Pt < 40 MeV/c (disabled by default) 
    3) when apply eta cut and selection of beam particles  do not assume that primary vertex is at Z axis.