Fix TrackingVertex->TrackingParticle links in premixing

[makortel]

Validation of 760pre6 revealed that the TrackingVertex->TrackingParticle links were not working https://hypernews.cern.ch/HyperNews/CMS/get/relval/4162/15.html. This PR fixes the problem.

Tested in 760pre6 with wfs 250199+250202.0 (latter manually configured to use the output of the former for premixed pileup events). Expecting differences in all validation plots using the TrackingParticles via TrackingVertex, e.g. in primary vertex validation

• anything vs. number of TrackingParticles and their sum(pT^2)
• anything related to merged vertices (e.g. resolutions)

[makortel]

@mdhildreth The issue in 760pre6 RelVals turned indeed out to be "no TrackingParticleRefs in TrackingVertex", and this seems to fix the issue (Refs appear, I didn't compare to classical mixing). Do you agree on the fix?

[cmsbuild]

A new Pull Request was created by @makortel (Matti Kortelainen) for CMSSW_7_6_X.

Fix TrackingVertex->TrackingParticle links in premixing

It involves the following packages:

SimGeneral/DataMixingModule

@cmsbuild, @civanch, @mdhildreth can you please review it and eventually sign? Thanks.
@wmtan this is something you requested to watch as well.
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@Degano you are the release manager for this.
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[civanch]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/8961/console

[cmsbuild]

-1
Tested at: 75a2164
When I ran the RelVals I found an error in the following worklfows:
1003.0 step1

DAS Error

you can see the results of the tests here:
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-11854/8961/summary.html

[civanch]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/9075/console
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/9078/console

[cmsbuild]

The jenkins tests job failed, please try again.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/9075/console

[cmsbuild]

+1
Tested at: 75a2164
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-11854/9078/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-11854/9078/summary.html

[civanch]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_6_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar

[davidlange6]

+1