Add a Pythia8 gun for displaced particles (flat in Pt and Dxy) [backport of #33832 to 10_6_X]

[mbluj]

PR description:

As title says: the PR adds a Pythia8 gun for displaced particles which are generated flat in Pt and Dxy in ranges provided by a user. This new gun enables generation of displaced unstable particles which is not possible with already existing FlatRandomPtAndDxyGunProducer.

Technically, the PR migrates an algorithm present in the FlatRandomPtAndDxyGunProducer to a new module based on a generic Py8 gun (Py8GunBase).

Backport of #33832

PR validation:

It was validated with a custom workflow that the newly added module behaves as expected. Test configuration (GeneratorInterface/Pythia8Interface/test/Py8PtAndDxyGun_displacedTaus_cfg.py) is part of this PR.
The change does not affect any existing workflows by construction (a new "generator source" added).

if this PR is a backport please specify the original PR and why you need to backport that PR:

Backport of #33832 for a MC UL sample production.

[cmsbuild]

A new Pull Request was created by @mbluj for CMSSW_10_6_X.

It involves the following packages:

GeneratorInterface/Pythia8Interface

@SiewYan, @mkirsano, @cmsbuild, @GurpreetSinghChahal, @agrohsje, @alberto-sanchez can you please review it and eventually sign? Thanks.
@alberto-sanchez, @mkirsano this is something you requested to watch as well.
@silviodonato, @dpiparo, @qliphy you are the release manager for this.

cms-bot commands are listed here

[agrohsje]

please test

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-53dfad/16203/summary.html
COMMIT: c33d978
CMSSW: CMSSW_10_6_X_2021-06-23-1100/slc7_amd64_gcc700
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmssw/34227/16203/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

Summary:

• No significant changes to the logs found
• Reco comparison results: 0 differences found in the comparisons
• DQMHistoTests: Total files compared: 35
• DQMHistoTests: Total histograms compared: 3215686
• DQMHistoTests: Total failures: 1
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3215351
• DQMHistoTests: Total skipped: 334
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 34 files compared)
• Checked 143 log files, 29 edm output root files, 35 DQM output files
• TriggerResults: no differences found

[agrohsje]

+generators

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_10_6_X IBs (tests are also fine) and once validation in the development release cycle CMSSW_12_0_X is complete. This pull request will now be reviewed by the release team before it's merged. @silviodonato, @dpiparo, @qliphy (and backports should be raised in the release meeting by the corresponding L2)

[qliphy]

+1