Add variable fPruneSelect which is a flag for which prune
    test selected the good track.

    Put into tree as tr.PruneSelect
    PruneSelect = -1 if no track found
    PruneSelect = 0 if only one track found
    PruneSelect > 0 when more then one track found.

    Method BestTrackUsingPrune goes through the tests in order and
     if a track fails the test then it is flagged as bad and not
     tested in the subsequent tests.
     When only one track is left then fPruneSelect is set to the number of
     the test.


  fPruneSelect = 1  if xptar
  fPruneSelect = 2  if yptar
  fPruneSelect = 3  if ytar
  fPruneSelect = 4  if delta
  fPruneSelect = 5  if track beta
  fPruneSelect = 6  if track reduced degrees of freedom (NHits-4)
  fPruneSelect = 7  if Number of PMT
  fPruneSelect = 8  if track beta chi2
  fPruneSelect = 9  if track focal plane time
  fPruneSelect = 10 if track hit S2Y plane
  fPruneSelect = 11 if track hit S2X plane
  fPruneSelect = 12 if track chisq