THcHallCSpectrometer
  1) Add variables fMispointing_x and fMispointing_y
    which are filled by parameters hmispointing_x, hmispointing_y
      in PARAM/HMS/GEN/hmsflags.param and pmispointing_x, pmispointing_y
      in PARAM/SHMS/GEN/shmsflags.param .
   The offsets are used in THcExtTarCor and THaReactionPoint
  2) Modify argument to method CalculateTargetQuantities to be xtar
     instead of gbeam_y .

THcExtTarCor
   1) add call to get spectrometer pointing offsets
   2) The mispointing_x is added to xtar
   3) Set xtar to start as -beam_y = - vertex(1)+mispointing_x for first call to CalculateTargetQuantities
      After first call the xtar is recalculated using the returned xptar
      and a second call is done to get the target xptar,ytar,yptar and delta.
    4) Fill golden track with final xtar