Modify THcDC and THcHallCSpectrometer

The purpose is to add a call in THcHallCSpectrometer:TrackCalc
to fill variables in THcDC for the golden track.

THcHallCSpectrometer.h
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1) Add object THcDC* fDC
2) Eliminate object fShower since it was not being used.

THcHallCSpectrometer.cxx
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1) in ReadDatabase method cast fDC in detector "dc" is defined
2) in FindVertices method add call
    to fDC->SetFocalPlaneBestTrack(hit_gold_track)
    where hit_gold_track is the HaTracks array index
    of the golden track

THcDC.h
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1) Add method SetFocalPlaneBestTrack

THcDC.cxx
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1) Eliminate filling of best focal plane quantities
   in CoarseTrack which defined "best" by lowest chi2
2) Create method SetFocalPlaneBestTrack
   a)fills the best focal plane quantities using the golden track
   b) fills fResiduals using golden track