Backport of POWHEG EmissionVeto1 updates

[AndrewLevin]

This is a backport of my recent pull request #24763 which updates the POWHEG EmissionVeto1 userhook for the WGamma special case. It was also necessary to backport the pull request #23469 since #24763 is built on top of #23469. @grohsjea and @agrohsje, please check that I backported your pull request properly.

[cmsbuild]

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

It involves the following packages:

GeneratorInterface/Pythia8Interface

@alberto-sanchez, @cmsbuild, @qliphy, @perrozzi, @efeyazgan can you please review it and eventually sign? Thanks.
@alberto-sanchez, @agrohsje, @mkirsano this is something you requested to watch as well.
@davidlange6, @slava77, @fabiocos you are the release manager for this.

cms-bot commands are listed here

[efeyazgan]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/30931/console Started: 2018/10/08 19:06

[cmsbuild]

+1
Tested at: e08775d
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-24828/30931/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-24828/30931/summary.html

Comparison Summary:

• No significant changes to the logs found
• Reco comparison results: 0 differences found in the comparisons
• DQMHistoTests: Total files compared: 11
• DQMHistoTests: Total histograms compared: 682535
• DQMHistoTests: Total failures: 2672
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 679489
• DQMHistoTests: Total skipped: 374
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: -10 KiB( 10 files compared)
• Checked 44 log files, 4 edm output root files, 11 DQM output files

[efeyazgan]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_1_X IBs (tests are also fine) and once validation in the development release cycle CMSSW_10_3_X is complete. This pull request will now be reviewed by the release team before it's merged. @davidlange6, @slava77, @smuzaffar, @fabiocos (and backports should be raised in the release meeting by the corresponding L2)

[fabiocos]

@AndrewLevin please update the title with a more explicative sentence

[AndrewLevin]

done

[fabiocos]

+1