Change the way total cross section is computed in GenXSecAnalzyer and fix the print out in LHERunInfo.cc

[cmsbuild]

GenXSecAnalyzer: when computing the total cross section after jet matching, instead of multiplying the
jet matching efficiency with individual cross section first and adding up the cross section from each process
later, now we add the cross section before matching first and multiply this cross section with the total jet matching efficiency.

LHERunInfo.cc: previous version did not include the process that has zero jet matching efficiency when computing the total jet matching efficiency. It is fixed now.

The threshold of 1e-6 has been modified to zero too.
Automatically ported from CMSSW_7_4_X #7674

[cmsbuild]

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

Change the way total cross section is computed in GenXSecAnalzyer and fix the print out in LHERunInfo.cc

It involves the following packages:

GeneratorInterface/Core
GeneratorInterface/LHEInterface

@vciulli, @covarell, @thuer, @cmsbuild, @nclopezo, @bendavid can you please review it and eventually sign? Thanks.
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@nclopezo, @ktf you are the release manager for this.
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[cmsbuild]

+1
Tested at: df7b9c9
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-7788/2735/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-7788/2735/summary.html

[bendavid]

+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, @ktf, @smuzaffar

[davidlange6]

+1