Add new GenFilter for use by HI photon team (71X)

[harmonicoscillator]

backport of #12510

This gen filter for Pythia8 is meant to mimic the older one used with Pyquen, which can be found here:
https://github.com/cms-sw/cmssw/blob/CMSSW_7_5_X/GeneratorInterface/HiGenCommon/src/EcalGenEvtSelectorFrag.cc

@bi-ran has done some sanity checking between the old filter and new filter which can be found here:
https://twiki.cern.ch/twiki/pub/CMS/MITHIGAnalysis2015/MCFilters.pdf

We would like to include these filters in the 71X series to be used for HI MC pp reference-run production.

[cmsbuild]

A new Pull Request was created by @richard-cms (R. Alex Barbieri) for CMSSW_7_1_X.

It involves the following packages:

GeneratorInterface/GenFilters

@vciulli, @covarell, @thuer, @cmsbuild, @bendavid, @davidlange6, @govoni can you please review it and eventually sign? Thanks.
@mkirsano this is something you requested to watch as well.
@slava77, @Degano, @smuzaffar you are the release manager for this.

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[covarell]

Why is this needed in 7.1.X? Is it planned for use also in pp reference MC?

[harmonicoscillator]

@covarell Yes, we want to use these for pp reference run MC.

[covarell]

+1

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/10100/console

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_1_X IBs after it passes the integration tests. This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @Degano, @smuzaffar

[cmsbuild]

+1
Tested at: 43ff961
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12645/10100/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-12645/10100/summary.html

[davidlange6]

+1