fix: write single hit collection for Roman Pots and Off-Momentum Trackers

[wdconinc]

Briefly, what does this PR introduce?

About half a year ago (or longer), when switching fully from dd4pod to edm4hep, we had to put all hits in a dedicated collection instead of reusing the collections across subdetectors. This reverts the behavior as DD4hep versions 1.21 and later have fixed this issues.

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue #__)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
  • Benchmarks need updating:
    • https://eicweb.phy.anl.gov/EIC/benchmarks/detector_benchmarks/-/merge_requests/130
    • https://eicweb.phy.anl.gov/EIC/benchmarks/reconstruction_benchmarks/-/merge_requests/269
    • https://eicweb.phy.anl.gov/EIC/benchmarks/physics_benchmarks/-/merge_requests/179
• Documentation has been added / updated
• Changes have been communicated to collaborators:
  • Post on mattermost about this
  • Let @ajentsch know about this

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

Yes. This now requires DD4hep 1.21 or newer for correct behavior (more recent than LCG 101).

Does this PR change default behavior?

Yes. By default the hits collections for roman pots and off-momentum trackers will not be numbered anymore.

[ajentsch]

Hey @wdconinc , just for me to understand (sorry for coming to this late - I was on travel most of last week, and didn't get back to the office until Tuesday).

Will I still be able to access hits in various layers independently? The reconstruction will require that I use the hit information from layers 1+2 and layers 3+4 independently for the matrix calculations.

[wdconinc]

This shouldn't change anything for the algorithms you are using: you were accepting a collection of tracker hits before and that same collection should still contain the exact same hits. What changes is simply where those hits are combined: previously they were written as separate collections by ddsim and combined in a juggler algorithm. Now they are combined in ddsim and juggler doesn't need to do the combination anymore. The only algorithms that would be affected are, for example, those that explicitly accept a collection of hits for layers 1+2 and a separate collection of hits for layers 3+4. That does not apply to the FarForwardParticles or FarForwardParticlesOMD algorithms. I hope that makes sense.