Update for Reference time Differencee

Add methods to THcRawHit,THcRawTdcHit,
THcRawAdcHit,THcRawDCHit,THcRawHodoHit,THcRawShowerHit
 to be able to Set and Get the time difference between
good reference time pulse and the previous pulse
  if more than one pulse in channel for that event.

Modified THcHitList to Set the reference time difference
between good reference time pulse and the previous pulse.
If no previouse pulse then difference ==0.

Modified THcScintillatorPLane to have new tree variables
for each plane : PosTdcRefDiffTime,PosAdcRefDiffTime
NegTdcRefDiffTime,NegAdcRefDiffTime
so that one can see the time difference
between good reference time pulse and the previous pulse.