Backport RunningAverage and use for PFRecHitProducer reserve

[mark-grimes]

This backports edm::RunningAverage from 75X and uses it to guess the size of the PFRecHitProducer collection. There are a loads of plots on #9106 that show the benefit at 200 pileup.
#9106 applies shrink_to_fit on the collection afterwards as well, which might or might not be merged since it increases the peak memory. Using an accurate reserve however gives a clear benefit so I'll add this separately first.

[mark-grimes]

please test

Forgot to say - the unit test is slightly different because the version of GCC in 6_2_X_SLHC doesn't have thread_local.

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

A new Pull Request was created by @mark-grimes (Mark Grimes) for CMSSW_6_2_X_SLHC.

Backport RunningAverage and use for PFRecHitProducer reserve

It involves the following packages:

FWCore/Utilities
RecoParticleFlow/PFClusterProducer

@nclopezo, @cvuosalo, @cmsbuild, @Dr15Jones, @slava77 can you please review it and eventually sign? Thanks.
@mmarionncern, @wmtan, @makortel, @wddgit, @lgray, @Martin-Grunewald, @bachtis this is something you requested to watch as well.
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@fratnikov, @mark-grimes you are the release manager for this.
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[mark-grimes]

merge

Tests seem stuck, but I've tested privately.

[cmsbuild]

+1
Tested at: 85e61d7
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-9135/4793/summary.html