Spectra with Momentum

[Laurie Nevay]

We have kinetic energy, total energy and rigidity for the rebdsim spectra command but momentum would be useful for multi-species beamlines.

[Laurie Nevay]

This has been implemented in v1.7.4. The commands can be found here: http://www.pp.rhul.ac.uk/bdsim/manual/output_analysis.html#spectra-commands