Massive HGC Clustering speed/memory improvements

[lgray]

Clustering memory footprint reduced by multiple gigabytes, clustering itself now causes little memory blow-up, O(100 MB) or so, down from 3/4 gigabytes from using a dense matrix.
Clustering timing improved by a factor of ~100, i.e. hour/event -> 30s/event spent on clustering.
Clustering performance appears the same looking at a limited number of events. EM clusters are formed correctly, charged pions look fine, simple jet gun multi-particle response is as before.
No changes in energies.

With DQM the event time in TTBar + 140 PU is at ~5 minutes per event.
Is this OK for now?

NB: I had to turn off the gsf electron tracking since there's no notion of preselection for the HGC clusters. This was causing the electron seeding and track fitting to run very slowly.

@vandreev11 @pfs @amagnan @bsunanda

[cmsbuild]

A new Pull Request was created by @lgray (Lindsey Gray) for CMSSW_6_2_X_SLHC.

Massive HGC Clustering speed/memory improvements

It involves the following packages:

DataFormats/ParticleFlowCandidate
Fireworks/Calo
Fireworks/ParticleFlow
RecoParticleFlow/PFClusterProducer
RecoParticleFlow/PFProducer
SLHCUpgradeSimulations/Configuration

@civanch, @nclopezo, @mdhildreth, @cmsbuild, @alja, @Dr15Jones, @StoyanStoynev, @slava77, @Degano, @ktf can you please review it and eventually sign? Thanks.
@alja, @bachtis 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.
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@fratnikov, @mark-grimes you are the release manager for this.
You can merge this pull request by typing 'merge' in the first line of your comment.

[mark-grimes]

merge

All tests fine.