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