Substantial clean-up of TrackerHitAssociator

Per request from David Lange, remove all unused code.
Several rewrites to reduce duplication of code.
Streamline collection read-in (thanks to Danek).
Add the capability to specify PSimHit collections, rather than crossing-frame collections, via the ROUList in the config parameter set.
No other change in functionality; results match those of the previous PR for this class.