TrackingParticle updates for pre mixing

[mdhildreth]

Added capability of saving TrackingParticles and TrackingVertices from Pileup BXs to PreMixing framework. DIGIPREMIX:pdigi_valid is now enabled for this from the cmsDriver.py command line.

[mdhildreth]

+1

[mdhildreth]

RelVal scripts will need to be changed in order to have this as the default for validation. Should this be included here?

[cmsbuild]

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

TrackingParticle updates for pre mixing

It involves the following packages:

Configuration/Applications
Configuration/EventContent
SLHCUpgradeSimulations/Configuration
SimGeneral/DataMixingModule
SimGeneral/MixingModule

@cmsbuild, @franzoni, @davidlange6 can you please review it and eventually sign? Thanks.
@ghellwig, @makortel, @Martin-Grunewald, @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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