Fix setup of signal particle gun directory name based on the beam energy.

From Gauss v49r13 the way how beam momentum in GeV was set interfered with signal particle guns as there was no protection against filling information with non-sense if running pp signal particle gun. I added some protection to change things only when one of the beams different from the proton. I did check that this fixes signal particle gun but did not try to verify that Hijing will work properly. What I did might not be best, perhaps the alternative of using completely independent variables for two purposes might be a better choice.

Closes LHCBGAUSS-1773