FIX: Use mean aggregation for InterRDF bins

[raulloiscuns]

FIX: update bin calculation to use ndarray_mean in aggregator of InterRDF and correct InterRDF tests to correctly support multiple frames

Changes made in this Pull Request:

• Fixed parallel aggregation for bins: Updated the ResultsGroup aggregator in InterRDF to use ndarray_mean instead of ndarray_sum for bins.
• Added multi-frame support to tests: Modified the sels fixture in test_rdf.py to generate a mock trajectory with multiple frames.
• Adjusted test_double_run: Removed the parallel backend parametrization for test_double_run, restricting it to the serial backend.
• Renamed duplicated test_norm

LLM / AI generated code disclosure

LLMs or other AI-powered tools (beyond simple IDE use cases) were used in this contribution: no

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS? (If it is not, add it!)
• LLM/AI disclosure was updated.

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

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📚 Documentation preview 📚: https://mdanalysis--5344.org.readthedocs.build/en/5344/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.83%. Comparing base (d150bfe) to head (aa9ddfb).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5344      +/-   ##
===========================================
- Coverage    93.84%   93.83%   -0.01%     
===========================================
  Files          182      182              
  Lines        22492    22493       +1     
  Branches      3199     3199              
===========================================
  Hits         21107    21107              
- Misses         923      924       +1     
  Partials       462      462              

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[orbeckst]

@raulloiscuns could you please open an issue that briefly summarizes the bug and then we can link this PR to the issue? This is important for anything that may lead to incorrect results. PRs come and go but issues are the important record of a problem.

For the PR, could you please check the formatting — black is unhappy https://github.com/MDAnalysis/mdanalysis/actions/runs/23625862074/job/68814847710?pr=5344 ?

Please also update CHANGELOG.

Thanks!