Fix: #5496 Incorrect InterRDF_s aggregator

[BradyAJohnston]

Fixes #5496

Tests were previously passing because we were only testing for 2 pairs, whereas this issue showed up with 1 pair and 3+ pairs.

The custom aggregator is now much simpler. The previous solution was over-engineered (and wrong), as we only ever have to deal with list[list[arr, ...], ...] and never anything more deeply nested.

Changes made in this Pull Request:

• nested_array_sum sums the count arrays pair by pair across workers; it previously returned two arrays, whatever the number of pairs.
• added test_n_pairs for 1, 2 and 3 pairs on all backends

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!)
• I have read and understand the current AI Policy
• LLM/AI disclosure was updated.

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

[read-the-docs-community]

Documentation build overview

📚 MDAnalysis | 🛠️ Build #35213497 | 📁 Comparing bc911ae against latest (94b4461)

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3 files changed

± documentation_pages/analysis/rdf.html
± documentation_pages/analysis/wbridge_analysis.html
± _modules/MDAnalysis/analysis/rdf.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.88%. Comparing base (94b4461) to head (bc911ae).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5497      +/-   ##
===========================================
- Coverage    93.88%   93.88%   -0.01%     
===========================================
  Files          184      184              
  Lines        22640    22631       -9     
  Branches      3217     3215       -2     
===========================================
- Hits         21256    21247       -9     
  Misses         920      920              
  Partials       464      464              

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