Substantial clean-up of TrackerHitAssociator

[wmtford]

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.

[cmsbuild]

A new Pull Request was created by @wmtford for CMSSW_7_1_X.

Substantial clean-up of TrackerHitAssociator

It involves the following packages:

SimTracker/TrackerHitAssociation

@cmsbuild, @civanch, @Degano, @mdhildreth, @nclopezo can you please review it and eventually sign? Thanks.
@cerati, @GiacomoSguazzoni, @rovere, @venturia this is something you requested to watch as well.
You can sign-off by replying to this message having '+1' in the first line of your reply.
You can reject by replying to this message having '-1' in the first line of your reply.
@nclopezo, @ktf you are the release manager for this.
You can merge this pull request by typing 'merge' in the first line of your comment.

[cmsbuild]

+1
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-3021/668/summary.html

[civanch]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_1_X IBs unless changes (tests are also fine). @nclopezo, @ktf can you please take care of it?