Add method THcHallCSpectrometer::CalculateTargetQuantities

This method calculates the target quantities using the focal
plane quantities and the optics matrix. Previously
this was done in THcHallCSpectrometer::FindVertices.
Now in THcHallCSpectrometer::FindVertices there is
a call to CalculateTargetQuantities


This method will be called by THcExtTarCor
to do the xtar correction.