object to instantiate a G4VSensitiveDetector 
    class
    ->  allows to use pre-existing SD classes 
    -> changed macro DECLARE_SENSITIVEDETECTOR to
       create a factory, thus replace:
       //    typedef Geant4GenericSD<TrackerCombine>  Geant4TrackerCombine;
       typedef SimpleG4SDFactory< Geant4GenericSD<TrackerCombine> >  Geant4TrackerCombine;