Partially implement THcDC::TrackFit, modelled after h_track_fit.

  Uses ROOT matrix math instead of solve_four_by_four
  Nothing is done with found tracks yet.  Will need to stuff these into
     podd style tracks.  (Or a class inherited from podd style tracks.)
  Residuals not saved anywhere
  Single stub mode not yet implemented
  No printing out of trackfit results (hdebugtrackprint)