Keep sensitive detector type constant. Add property to Geant4Kernel if special/user defined G4VSensitiveDetector instances should be created

Add test to dump hits from MultiSegment-MultiCollections example. Investigate issue: https://github.com/AIDASoft/DD4hep/issues/141 for details

Support for multi-threaded Geant4 applications using DDG4Details are described in the release notes.