Regression test for CLOUDSC2

[reuterbal]

This adds a regression test for CLOUDSC2 processing, and enables it as part of the Github actions CI tests.
Temporarily, it points to a branch for CLOUDSC2, which has a pending PR for integration: ecmwf-ifs/dwarf-p-cloudsc2-tl-ad#14

The PR is filed against the new Scheduler branch but doesn't have to be integrated now and can be left dangling for the moment.

Processing works fine and results are correct with GNU on host-side (GPU evaluation is tbd), however, in the Actions runner I'm seeing randomly massive deviations for steps 4 and 6 in the TL variant. No clue what is going on there.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/230/index.html

[codecov]

Codecov Report

Attention: Patch coverage is 27.02703% with 54 lines in your changes missing coverage. Please review.

Project coverage is 95.90%. Comparing base (f3b7327) to head (4d87ec6).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
loki/transformations/tests/test_cloudsc2_tl_ad.py	27.02%	54 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
- Coverage   96.03%   95.90%   -0.14%     
==========================================
  Files         197      198       +1     
  Lines       39019    39093      +74     
==========================================
+ Hits        37472    37491      +19     
- Misses       1547     1602      +55     

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	95.89% <27.02%> (-0.14%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[reuterbal]

I have rebased this over #408 to exploit the update to the runner images (and thus compilers), now the weird issues are gone 🎉
I think this is finally ready @mlange05

Note that the last commit points to a temporary cloudsc2 branch. The commit can be removed once ecmwf-ifs/dwarf-p-cloudsc2-tl-ad#15 has been merged.

[mlange05]

This looks fantastic, and is finally passing tests! Really great stuff, GTG from me.

Note, the coverage failure is unavoidable, as regression tests do not trigger coverage tests. I'll override this on merge.