Protect against infinite loops in gen status flag code

[bendavid]

Some generators (ie sherpa) produce circular mother-daughter links in the generator truth. This caused infinite loops when filling gen status flags. (at either gen-sim or digi-reco time).
Protections have been added for this now.

Because the protections require some dynamically allocated space to keep track of the particles, implementing this efficiently required transitioning from a namespace to a class for MCTruthHelper.

Verified that there are no changes in flags for pythia8 samples where circular relations were not present.

This is a generalized fix which supersedes the more specific protection in
#9272

[cmsbuild]

A new Pull Request was created by @bendavid (Josh Bendavid) for CMSSW_7_5_X.

Protect against infinite loops in gen status flag code

It involves the following packages:

PhysicsTools/HepMCCandAlgos

@cmsbuild, @vadler, @monttj can you please review it and eventually sign? Thanks.
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[davidlange6]

@cmsbuild, please test

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

+1
Tested at: 02e64fa
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-9489/5350/summary.html

[monttj]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_5_X IBs unless changes (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar

[davidlange6]

+1