Alpha Particle Production in SingleGen_module

[mantheys]

Currently, it is impossible to produce alpha particles correctly with the SingleGen_module. The particle gun will run if the alpha pdg (1000020040) is provided, but the output will not follow the expected parameters. This issue was already identified and potentially solved in the RadioGen_module. https://github.com/LArSoft/larsim/blob/develop/larsim/EventGenerator/RadioGen_module.cc As it is described there in the code "alpha particles need a little help since they're not in the TDatabasePDG table so don't rely so heavily on default arguments to the MCParticle constructor."