McXtrace MCPL_output: Should we add phase as uservar?

[willend]

Working on #2019 preparing for #2018 I realised that MCPL_output in McXtrace sets up for using the X-ray phase phi but never uses it for anything...

I think that as a minimum it would be reasonable to (maybe via a specific component input-flag?) let MCPL_output use the userflag field for this value? Given a relevant comment-string a corresponding MCPL_input could then pick up the phase for a subsequent run...