Add ability to change G4 production cuts

Gauss().DeltaRays = False

Gauss().ProductionCuts['electron'] = Value

Gauss().DeltaRays = False

Hi @pgriffit, looking at the properties, there is another one ```DeltaRays``` that in fact is related to the value of the ```ProductionCuts``` for electrons. It would be good to implement a check (and a warning) that ``` Gauss().DeltaRays = False ``` could only be set if there is no property set ``` Gauss().ProductionCuts['electron'] = Value ``` and also have the code that applies the ``` Gauss().DeltaRays = False ``` use the new way of setting the value of the production cuts for electrons. Cheers, Gloria