FastSim: fix trackref in ConversionTrackCollection

[lveldere]

• the original ConversionTrackCollection is copied. In the copy, reference to tracks in the before-mixing track collection are replaced with references to the after-mixing track collection
• the before-mixing track collection is dropped from AODSIM, which is justified by the above
• should be considered for backporting to 74X as it reduces the AODSIM event content

[lveldere]

please test

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

A new Pull Request was created by @lveldere for CMSSW_7_5_X.

FastSim: fix trackref in ConversionTrackCollection

It involves the following packages:

FastSimulation/Configuration
FastSimulation/Tracking

@ssekmen, @nclopezo, @lveldere, @civanch, @mdhildreth, @cmsbuild can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @matt-komm this is something you requested to watch as well.
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@nclopezo you are the release manager for this.
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[cmsbuild]

+1
Tested at: db75537
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-9202/7759/summary.html

[lveldere]

+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, @nclopezo, @smuzaffar

[davidlange6]

+1

[lveldere]

@davidlange6
I'm checking how much effect this has on the AODSIM file size.
If it's significant enough, I'll prepare a backport for 74X today.

[lveldere]

Pretty obvious this needs to be backported:

TTbar, 13 TeV, no PU, AODSIM


• FullSim: 136 kB / event (3000 events, RelVal)
• 
FastSim before: 210 kB / event ( 100 events, private)
• 
FastSim after: 136 kB /event (100 events, private)

The absolute gain (~80 kB / event) is expected to be constant versus PU