(Backport) Provide forward compatibility to HepMC::GenEvent reading

[srimanob]

PR description:

To provide forward compatibility to HepMC::GenEvent reading. This is used for ultra-legacy processing with 2016 (CMSSW_8_0) HLT.

PR validation:

CMSSW_8_0_X_2019-04-07-0000 with this PR on top
cmsDriver.py HLT --conditions 80X_mcRun2_asymptotic_2016_TrancheIV_v6 -s HLT:@frozen2016 --datatier GEN-SIM-DIGI-RAW-HLTDEBUG -n -1 --era Run2_2016 --eventcontent FEVTDEBUGHLT --filein file:step2.root --python step2a_HLT.py --fileout step2a.root --no_exec --customise_commands 'process.source.bypassVersionCheck = cms.untracked.bool(True)' --customise Configuration/DataProcessing/Utils.addMonitoring --outputCommand 'keep *_mix_*_*','keep *_genPUProtons_*_*' --inputCommand 'keep *','drop *_*_BMTF_*','drop *PixelFEDChannel*_*_*_*' --mc

if this PR is a backport please specify the original PR:

backport of #26211
Thx @Dr15Jones

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• verify that the PR is really intended for the chosen branch
• verify that changes follow CMS Naming, Coding, And Style Rules
• verify that the PR passes the basic test procedure suggested in the CMSSW PR instructions

[cmsbuild]

A new Pull Request was created by @srimanob (Phat Srimanobhas) for CMSSW_8_0_X.

It involves the following packages:

SimDataFormats/GeneratorProducts

@alberto-sanchez, @cmsbuild, @efeyazgan, @perrozzi, @qliphy can you please review it and eventually sign? Thanks.
@rovere 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

[Dr15Jones]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/34123/console Started: 2019/04/10 14:57

[cmsbuild]

+1
Tested at: 04063cc
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-26422/34123/summary.html

[cmsbuild]

Comparison job queued.

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-26422/34123/summary.html

Comparison Summary:

• No significant changes to the logs found
• Reco comparison results: 0 differences found in the comparisons
• DQMHistoTests: Total files compared: 15
• DQMHistoTests: Total histograms compared: 1044669
• DQMHistoTests: Total failures: 942
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 1043619
• DQMHistoTests: Total skipped: 108
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: -14 KiB( 14 files compared)
• Checked 62 log files, 7 edm output root files, 15 DQM output files

[alberto-sanchez]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_8_0_X IBs (tests are also fine) and once validation in the development release cycle CMSSW_10_6_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]

+1