add originalXWGTUP() to LHEEvent and LHEEventProduct 62X_SLHC edition

[lgray]

Rebase of #2097 to CMSSW 62X_SLHC

Had to also bring in the commits from #1020, since they did not make it into the SLHC branch.

[cmsbuild]

A new Pull Request was created by @lgray (Lindsey Gray) for CMSSW_6_2_X_SLHC.

add originalXWGTUP() to LHEEvent and LHEEventProduct 62X_SLHC edition

It involves the following packages:

GeneratorInterface/LHEInterface
SimDataFormats/GeneratorProducts

@vciulli, @cmsbuild, @Degano, @bendavid, @nclopezo can you please review it and eventually sign? Thanks.
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@andersonjacob, @kknb1056 you are the release manager for this.
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[cmsbuild]

Pull request #2101 was updated. @vciulli, @cmsbuild, @Degano, @bendavid, @nclopezo can you please check and sign again.

[mark-grimes]

Builds fine and tests pass. However, I don't think the tests I ran test any of these changes. I put a throw exception at a random point in LHEReader.cc and the tests all still pass.
@lgray, you say in request 2097 (the same thing but on CMSSW_7) that you've checked this by writing the file read in, have you tried this in 6_2_X_SLHC or is there likely to be no difference? Otherwise, do you know of a test I can run that will check this? I tried with:

runTheMatrix.py -j 6 -w upgrade -l 3300,3400,4100,4400,40001,50002,60002,60001,4502,4500,5001,15001,50001

I'll keep an eye on 2097 and discuss this at the release planning meeting on Tuesday.

[lgray]

@mark-grimes Matrix tests will not catch any issue with this. The LHE reader and the capability to save weights was already tested and validated in 70X/62X/53X.

[mark-grimes]

merge

Discussed in the release planning meeting and agreed to accept.