[Halld-offline] RF Bunch Selection Added

[Paul Mattione]

I've added RF bunch selection to the PID code.  That way it no longer assumes that the event start time is zero (although right now the "measured" RF time is zero).  The routines assume high-luminosity running (i.e. too many tagger photons to distinguish the RF bunch).

How RF bunch selection works for an event (creating a single DEventRFBunch object):
1) The measured RF time and variance are assumed to be zero (until actual measured values are introduced).
2) In DEventRFBunch_factory: loop over DTrackTimeBased objects, determine the matching start counter hits, propagate the start counter times to the target center and select the closest-matching RF bunch (+/- multiples of the RF bunch frequency).
Note: tracks with bad vertex-z are ignored, and tracks with low track-reconstruction FOM are ignored unless there's no good tracks. 
Note: if there are no start counter hits then the track times from time-based tracking is used. 
3) Vote: select the RF bunch that matches the most tracks (tie broken by highest average tracking FOM).

How DChargedTrackHypothesis PID FOM calculation works:
1) If a track hypothesis has a start counter hit, select the RF bunch that best matches it (propagate start counter time to target center) (different hypotheses can match to different RF bunches)
Note: doing 1) allows the correct PID to be determined for background tracks originating from nearby beam buckets. 
2) If it does NOT have a start counter hit, then the DEventRFBunch time is used.
3) Compare the TOF/BCAL/etc. time to the RF time to calculate FOM.  

How DNeutralParticleHypothesis PID FOM calculation works:
1) Propagate shower times to the target center, compare them to the DEventRFBunch time. 

 - Paul