fix handling of air/iron in DD4hep MF construction

[namapane]

PR description:

MF yoke volumes are labelled as being iron for the purpose of track propagation in SHP . This was broken in DD4hep because the label is based on material name, which is reported with an unexpected "materials:" namespace in DD4hep.
To avoid this, the distinction is now made based on material density.

This is the origin of a discrepancy that has been observed in the DD4hep vs DDD validation for STA muons.

PR validation:

Checked that the isIron flag is now assigned correctly.

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-33593/22405

• This PR adds an extra 16KB to repository

[cmsbuild]

A new Pull Request was created by @namapane (Nicola Amapane) for master.

It involves the following packages:

MagneticField/GeomBuilder

@perrotta, @jpata, @cmsbuild, @slava77 can you please review it and eventually sign? Thanks.
@silviodonato, @dpiparo, @qliphy you are the release manager for this.

cms-bot commands are listed here

[ianna]

please test

[civanch]

@namapane , I think it is a correct fix.

[cvuosalo]

@namapane Thanks for finding this. I have confirmed that this case was the only one in CMSSW where the material name was compared without considering the namespace.
Using the density is a good fix. The alternative would be to strip any namespace from the material name with dd4hep::dd::noNamespace().

[civanch]

@cvuosalo , I would strip namespaces from materials but not in this PR. The check on density is much better than any check on name.

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-0338be/14758/summary.html
COMMIT: 44a0df9
CMSSW: CMSSW_12_0_X_2021-04-29-2300/slc7_amd64_gcc900
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmssw/33593/14758/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: 4 differences found in the comparisons
• DQMHistoTests: Total files compared: 37
• DQMHistoTests: Total histograms compared: 2662646
• DQMHistoTests: Total failures: 7
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 2662617
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 36 files compared)
• Checked 155 log files, 37 edm output root files, 37 DQM output files
• TriggerResults: no differences found

[cvuosalo]

+geometry

[slava77]

type bugfix

[slava77]

+reconstruction

for #33593 44a0df9

• code changes are in line with the PR description and some preceding discussion by email
• jenkins tests pass and comparisons with the baseline show no differences (DD4HEP workflow was disabled in 29-1100 by [DD4he] Disable DD4hep workflow 11634.911 in PR tests while it produces unstable results #33568 )
• the signoff is based on the code change conceptually and is also confirmed by comparisons in the 11_3_X backport where the workflow 11634.911 is still enabled fix handling of air/iron in DD4hep MF construction #33594 (comment)

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs (tests are also fine). 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