Add option to save initial vertex collection (forward port of #8975)

[mark-grimes]

This was an option added to 6_2_X_SLHC to save a TrackingVertex collection of just the initial interactions for each signal and pileup event. This was required for fast timing studies. I'm not sure how useful this would be in 75X but I thought I'd submit the PR and let you decide. More details on #8975.

The option is off by default, to turn in it on requires

process.mix.digitizers.mergedtruth.createInitialVertexCollection = True
process.FEVTDEBUGHLToutput.outputCommands.append('keep TrackingVertexs_mix_InitialVertices_*')

added to the digi config file. If on then at 0 pileup it adds 2980.5 bytes/event uncompressed, 695.2 bytes/event compressed (using workflow 38.0). The figure quoted on #8975 is for 140 pileup, I'd expect it to be roughly proportional to pileup.

[cmsbuild]

A new Pull Request was created by @mark-grimes (Mark Grimes) for CMSSW_7_5_X.

Add option to save initial vertex collection (forward port of #8975)

It involves the following packages:

SimGeneral/MixingModule
SimGeneral/TrackingAnalysis

@cmsbuild, @civanch, @nclopezo, @mdhildreth can you please review it and eventually sign? Thanks.
@wmtan, @makortel, @abbiendi, @GiacomoSguazzoni, @jhgoh, @VinInn, @appeltel, @rovere, @cerati, @trocino, @dgulhan, @battibass this is something you requested to watch as well.
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[cmsbuild]

+1
Tested at: 430a8ce
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8989/4586/summary.html

[civanch]

+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

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8989/4586/summary.html

[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