Make physics processes in parallel worlds optional
This extends the standard configuration of parallel worlds to accept no physics processes. This feature is needed in fast simulation studies using the CaloChallenge setup !146 (merged).
Parallel worlds give the possibility to add extra volumes on top of the current geometry and redirect some physics processes to these volumes. More info: https://gaussino.docs.cern.ch/master/configuration/geometry.html#parallel-geometry.
Adding this feature, will allow us to add volumes and sensitive detectors on top of existing ones and at the same telling Geant4 to ignore the physics processes in them. The sensitive detector class would have to be manually triggered in this case (for example via CustomSimulation hooks).