+ 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

Add component to allow the setup of the magnetic field tracking in Geant4 from python. See DDG4/examples/CLICSidSimu.py for the usage.

   -> added operators ==/!= for map<>::iterator

   over maps on macos 

First implementation of dipole field, HepMC reader for DDG4 and fix to import constants into DDG4 python