Main change was to allow the beam X and Y positions and angle
at the target to be read in through parameters instead of
always using the EPICS data. This gives added flexibility.

If either of the beam position parameters (gbeam_xoff or
gbeam_yoff) are read-in as parameters, then the code uses
the read-in parameters as the BPM position at target. If
there are other beam position or angle parameter that are
not read in then they are set to zero.


1) Modify THcRaster.h
   a) Added variables for X and Y beam angles at target.
   b) Added variable fFlag_use_EPICS_bpm which is a flag
     that is determines whether the code uses the parameters
     or EPICS reads.

2) Modified THcRaster.cxx
   a) set default BPM z-positions to Fall 2018 survey.
   b) Modified Process method so that the BPM position/angle variables
     that are set by parameters are no longer recalculated. Just
     use the existing variables fXbpm_tar, fYbpm_tar, fXpbpm_tar
      and fYPbpm_tar throughout the method.