Modify THcExtTarCor to calculate xsieve, ysieve for spectrometer

1) THcExtTarCor.h
   a) add variables fxsieve and fysieve
   b) Added clear method so that fxsieve and fysieve could be set to kBig at start of each event.

2) THcExtTarCor.cxx
   a) Add variables extcor.xsieve and extcor.ysieve to tree
   b) Added clear method.
   c) Added calculation of xsieve and ysieve for golden track. The
      calculation depends on spectrometer. SHMS includes delta dependence
      when calculating ysieve.