Add variables fX_fp_best,fY_fp_best,fXp_fp_best,fYp_fp_best to THcDC.h
Modified THcDC::CoarseTrack so that the variables are filled with the
quantities from the track with the smallest chi-squared.
Modified THcDC::TrackFit so that the fResidual per plane is filled with
for the track with the lowest chi-squared.
Modified THcDC::DefineVariables so that access the variables as
dc.x_fp,dc.y_fp,dc.xp_fp and dc.yp_fp