Allow to use different sources of momentum spectrum in signal particle guns

Change to allow several different momentum spectra for a single particle in particle guns. This change will require a new release of PGunsData but otherwise should be rather transparent (it will require at runtime one more options file from PGunsData package).

This is a version for Sim09, I will prepare separate MR for Sim10, where I want to do it slightly differently as I there I can make also changes to DecFiles.

Resolves LHCBGAUSS-1864