Add PDF weight helper tool (80x, updated)

[bendavid]

To be used at analysis level to apply the mc2hessian procedure on the fly for analysis needing hessian-type uncertainties from existing samples with mc replica-like weights.

(replaces #12787)

[cmsbuild]

A new Pull Request was created by @bendavid (Josh Bendavid) for CMSSW_8_0_X.

It involves the following packages:

PhysicsTools/HepMCCandAlgos

@cmsbuild, @vadler, @davidlange6, @monttj can you please review it and eventually sign? Thanks.
@slava77, @Degano, @smuzaffar you are the release manager for this.

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[bendavid]

@davidlange6 do you also want me to change the legacy class inheritance in the corresponding 74x and 76x backports?

[davidlange6]

@cmsbuild, please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/10687/console

[cmsbuild]

+1
Tested at: f56cfdf
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-13019/10687/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-13019/10687/summary.html

[davidlange6]

+1
@bendavid - would be best to have the back ports consistent with this one -yes. Thanks