Add missing implementation of VolumeType() method in G4MyPhysicalVolume class
G4MyPhysicalVolume class is implemented in DerivedG4PhysicalVolume.h file. The added method implements the corresponding pure virtual method of G4VPhysicalVolume class, to allow instanciation of G4MyPhysicalVolume objects.
This commit fix the compilation of r22.0.9 with Geant4.10.6 tag v10.6.0.1