Additional Geant4 Physics Lists

Current implementation of the DepositionGeant4 module only allows the base physics lists provided by Geant4. The method proposed here would allow additional physics lists to be implemented in the allpix2 framework outside of the default lists.

In particular, the MicroElec physics lists could be quite useful for detector simulation, since these are track structure models and hence offer greater spatial resolution at the consequence of extended simulation times.