Fix setting cross section in Rivet analaysis

[vciulli]

Fix a problem with setting the cross section from the python config

[cmsbuild]

A new Pull Request was created by @vciulli (Vitaliano Ciulli) for CMSSW_8_1_X.

It involves the following packages:

GeneratorInterface/RivetInterface

@covarell, @perrozzi, @thuer, @cmsbuild, @davidlange6, @govoni can you please review it and eventually sign? Thanks.
@agrohsje, @mkirsano this is something you requested to watch as well.
@slava77, @smuzaffar you are the release manager for this.

cms-bot commands are list here #13028

[perrozzi]

+1

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/14611/console

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_8_1_X IBs after it passes the integration tests. This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @smuzaffar

[cmsbuild]

+1
Tested at: 37ef0a7
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-15462/14611/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-15462/14611/summary.html

[davidlange6]

+1