Deposition: allow specifying particles by PDG code

Minor improvement: in addition to specifying the particle_type we should also allow specifying particle_code to select the particle via its PDG code - since we promote using PDG codes elsewhere in the framework it would be inconsistent not to support it here.

It should then be checked that not both parameters are specified.

Relevant call for Geant4 is G4int GetPDGEncoding().

Maybe @pschutze could address this in !53 (merged) since they go in the same module?