Adapt signal particle gun to allow multiple sources of momentum spectra.

Adapt signal particle gun to allow multiple sources of momentum spectra similar to what was done in Sim09. In DecFiles we add placeholder for energy and spectra source, which is updated during configuration based on actual beam energy and corresponding property for source of the spectra. The way how it is handled here is bit more transparent compared to Sim09 as here it is obvious in event type options that some parts of the path are placeholders.