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

TSystem::Load return > 0 if the library is already loaded. This is not an error. For some reason it was already loaded when I run ddsim in a singularity container. I have no idea why.

+ 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

Fix hundreds of std=c++11 compiler warningsAllow to change compiler settings in cmake/DD4hep.cmake valid for all packages.Watch out for CMAKE_CXX_FLAGS in cmake/DD4hep.cmake.Move to use Boost spirit::phoenix version 3. ---- Prepare for ROOT 6