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.

+ Minor changes to externilze the property/plugin mechanism.

I have never seen the compilation work and the error to load the library is usually irrecoverable[1]
It is just confusing for users when the compilation then also fails

[1] Like missing local libraries

   SIM.action.tracker = ("Geant4TrackerWeightedAction", {"HitPositionCombination" : 1 , "CollectSingleDeposits" :  False } )

This parameter is needed to not have single steps attempting to reach 1km (the
default, see
https://geant4.web.cern.ch/geant4/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/ch04s03.html
). This can happen for low energy curlers in the beam pipe vacuum.

Add this also to DDG4 setup and to documentation example for field
get default value in case it is not set

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

DDG4.py Output2ROOT: Only add '.root' extension if it is not already present. Consistent with lcio output