Add position computation with linear weights for pathological low energy clusters

[matteosan1]

This PR fixes a crash seen in prompt reco (run: 237686 lumi: 7 event: 33623).
The issue is due to very low energy cluster whose hits log-weights were all 0.

[cmsbuild]

A new Pull Request was created by @matteosan1 (Matteo Sani) for CMSSW_7_3_X.

Add position computation with linear weights for pathological low energy clusters

It involves the following packages:

RecoParticleFlow/PFClusterProducer

@cmsbuild, @cvuosalo, @nclopezo, @slava77 can you please review it and eventually sign? Thanks.
@mmarionncern, @bachtis, @lgray this is something you requested to watch as well.
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[bachtis]

Funny. @lgray , data dont like your logarithm :-) :-) :-)

[lgray]

@bachtis Considering that none of the zero suppression cuts have changed and the W0 is a relative cut on fraction of energy of a hit in a cluster.

Talking to Matteo this cluster was 40-50 hits with each hit being ~0.2 GeV or so before multiplying by the assigned fraction for the cluster. This could easily make the fractions go crazy and you could end up with very very small rechit energies.

Wouldn't worry about using vdt in realistic situations (as proven by CSA14 and PHYS14). Sounds like some detector noise issue. If so, we should study more deeply, but only if it turns up in collisions.

[slava77]

@cmsbuild please test

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

+1
Tested at: 265693b
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8530/3809/summary.html

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_3_X IBs unless changes (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @nclopezo, @ktf, @smuzaffar

[cvuosalo]

Additional tests with workflow 19.0_SingleGammaPt35+SingleGammaPt35 using ten times more statistics verify that this PR makes only insignificant changes to monitored quantities.

[davidlange6]

+1