fixed _conclude() for msd.py for non_linear frames

[spyke7]

Fixes #5100

Changes made in this Pull Request:

• checked the difference between times of frames to detect whether that is linear or non_linear (obviously for length > 1)
• updated test_msd.py to check non_linear is True, for uneven times (or frames)

Could have also used self.frames instead of self.times as well

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--5254.org.readthedocs.build/en/5254/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.82%. Comparing base (900d20b) to head (19ca69d).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5254      +/-   ##
===========================================
- Coverage    93.83%   93.82%   -0.01%     
===========================================
  Files          180      180              
  Lines        22473    22474       +1     
  Branches      3189     3189              
===========================================
- Hits         21088    21087       -1     
- Misses         923      924       +1     
- Partials       462      463       +1     

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