Add BPM epics X and Y histograms

1) Add BPM EPICS X and Y histograms
  to HMS/PRODUCTION/RASTER/hraster_histos.def
and SHMS/PRODUCTION/RASTER/praster_histos.def

2) Add script UTIL/BEAMLINE/plot_beam.C
  to plot the mean of the BPM histograms
   as a function of Z_beam to show the
   beam track on target.

3) Add call plot_beam.C to the production
 config files so that it is included in
the online analysis.