Activate all products needed by Sim/Reco-level mixing (in 7_4_X)

[wmtan]

Mixing in pileup at the Sim/Reco level, as done in fast simulation, does not work, because, while three different product types are mixed, only one of these product types is kept on input. This simple pull request adds one line of code to keep each of the other two needed product types. Note: This mixing currently works accidentally if only one pileup file is specified, because the first pileup file is opened at construction time, before the list of product types to be dropped is activated. So, Sim/Reco level mixing fails only if more than one pileup file is specified. This pull request fixes the failure.
This problem was reported by Jason Gran.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_4_X.

Activate all products needed by Sim/Reco-level mixing (in 7_4_X)

It involves the following packages:

SimGeneral/MixingModule

@cmsbuild, @civanch, @nclopezo, @mdhildreth can you please review it and eventually sign? Thanks.
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[ghost]

@cmsbuild please test

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

+1
Tested at: 0aa87f4
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-7438/2209/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-7438/2209/summary.html

[civanch]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_4_X IBs unless changes (tests are also fine). This pull request will be automatically merged.