fix: write local position in mm in CalorimeterHitsMerger

[wdconinc]

Briefly, what does this PR introduce?

This PR fixes an issue where the local position of merged hits is written in DD4hep units instead of EDM4eic units. The subsequent clustering expects the local position to be in EDM4eic units of course.

What kind of change does this PR introduce?

• Bug fix (issue: no clustering in backward HCal, https://chat.epic-eic.org/main/pl/pg6fwpimetgf8esmz4zfotwc3c)
• New feature (issue #__)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators @lkosarz

Does this PR introduce breaking changes? What changes might users need to make to their code?

No.

Does this PR change default behavior?

Fixes backward HCal clusters (should have changing number of clusters, distributed over 10x larger area).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

Capybara summary for PR 1598

• rec_dis_10x100_minQ2=0_craterlake
• rec_dis_10x100_minQ2=1000_craterlake_tracking_only
• rec_dis_18x275_minQ2=0_craterlake
• rec_dis_18x275_minQ2=1000_craterlake
• rec_dis_5x41_minQ2=0_craterlake
• rec_e_1GeV_20GeV_craterlake
• rec_pi_1GeV_20GeV_craterlake
  ^{_{Last updated 2024-08-23T19:14-04:00 145a9b4}}

[lkosarz]

Thanks! That fixed Reco clusters, but MC clusters are still wrong. I was suspecting the issue was with the algorithms.

[wdconinc]

Truth clustering was never intended to work with merged hits. Every other detector has moved on to real clusters so there's little incentive to fix truth clustering, I'm afraid.

[lkosarz]

I checked the hits associated to truth clusters and they look OK, well separated. I also had a quick look at truth clustering codes and don't see an obvious weak point. It would be good to have correct truth clusters later, but if others abandoned them we may do the same in the end. For now, they are not really an useful reference.