Add const-correct physical-value-only EtSumHelper and L1T demo

[mulhearn]

This PR replaces #13312.

This is for downstream consumers of the L1T output. A Helper class for interpreting somewhat confusing new format for the L1T EtSums is provided, along with demonstration code for all of L1T upgrade objects.

This version is const-correct and only accesses physical ET values.

[cmsbuild]

A new Pull Request was created by @mulhearn for CMSSW_8_0_X.

It involves the following packages:

DataFormats/L1Trigger
L1Trigger/L1TCommon

@cmsbuild, @mulhearn, @davidlange6 can you please review it and eventually sign? Thanks.
@Martin-Grunewald this is something you requested to watch as well.
@slava77, @Degano, @smuzaffar you are the release manager for this.

cms-bot commands are list here #13028

[mulhearn]

please test

[mulhearn]

+1

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/11270/console

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_8_0_X IBs after it passes the integration tests. This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @Degano, @smuzaffar

[cmsbuild]

+1
Tested at: f39fa16
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-13319/11270/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-13319/11270/summary.html