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

Implement issues https://github.com/AIDASoft/DD4hep/issues/373, https://github.com/AIDASoft/DD4hep/issues/371

Since gSystem.load() can return 1 if the library has already been
loaded, there are cases where requiring `result == 0` raises a false
exception.  This commit changes the required condition to be `result >=
0`

Fix example DDG4/examples/readHEPMC.py and allow access to STL containers from DD4hep python interface

+ Minor changes to externilze the property/plugin mechanism.