Fix from447

[tomcornelis]

Take the Pythia8Interface from CMSSW_4_4_7 and inplements it in branch CMSSW_4_2_X
This will fix the problem with weights in flat QCD samples

[cmsbuild]

A new Pull Request was created by @UAEDF-tomc (Tom Cornelis) for CMSSW_4_2_X.

Fix from447

It involves the following packages:

GeneratorInterface/Pythia8Interface

@vciulli, @nclopezo, @thuer, @cmsbuild, @bendavid, @Degano can you please review it and eventually sign? Thanks.
You can sign-off by replying to this message having '+1' in the first line of your reply.
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@smuzaffar, @nclopezo, @davidlt, @ktf you are the release manager for this.
You can merge this pull request by typing 'merge' in the first line of your comment.

[bendavid]

Is it possible to have a more minimal fix for this issue?

[tomcornelis]

I actually just copied the Pythia8Interface from CMSSW_4_4_7 to this CMSSW_4_2_X branch, as suggested by Mikhail in the hypernews thread. There are indeed a lot of lines changed in the Pythia8Hadronizer.cc, but I think most of them are for lay-out changes and additional options.
Maybe @mkirsano can comment on this?

[vciulli]

+1
I think it is safer to backport the full Pythia8Interface from 447, as it has been extensively tested there

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_4_2_X IBs unless changes or unless it breaks tests. @smuzaffar, @nclopezo, @davidlt, @ktf can you please take care of it?