ME0 pseudo digitizer: Fix the BX mixing window + enable bkg scaling with lumi

[cmsbuild]

It appears that in the ME0 pseudo digitizer module the TOF distribution corresponding to BX=0 in the BX window [-5,3] is centered at -7.5 ns. It should be at 18.5 ns (distance from ME0 to beam-spot). This pull request fixes that. The effect is visible when comparing the TOF from hits at PU200. See plots below.

The second part of this pull request enables to scale up/down the electron/neutron background for desired instantaneous lumi (reference is 5E34, double from config is in units of 1E34).

@nickmccoll @pietverwilligen

Automatically ported from CMSSW_8_1_X #16273 (original by @dildick).

[cmsbuild]

A new Pull Request was created by @cmsbuild for CMSSW_9_0_X.

It involves the following packages:

SimMuon/GEMDigitizer

@cmsbuild, @civanch, @mdhildreth, @davidlange6 can you please review it and eventually sign? Thanks.
@jhgoh this is something you requested to watch as well.
@slava77, @smuzaffar you are the release manager for this.

cms-bot commands are listed here #13028

[mdhildreth]

+1

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/16551/console

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_9_0_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: c0408bf
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-16632/16551/summary.html

[cmsbuild]

Comparison job queued.

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-16632/16551/summary.html

[davidlange6]

+1