Accept single Experiment in Fitter.fit()

[AndrewSazonov]

• Refactor Fitter.fit() and _residual_function() to accept a plain list of ExperimentBase objects instead of requiring an Experiments collection.
• Remove the dummy Experiments wrapper and the object.setattr hack in Analysis.fit() single-mode loop.
• Update _residual_function to iterate over the list directly. Update all callers (single-fit, joint-fit).
• Update unit and integration tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.77%. Comparing base (883c384) to head (0e42f9f).
⚠️ Report is 37 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #140      +/-   ##
===========================================
- Coverage    82.26%   81.77%   -0.49%     
===========================================
  Files           88      133      +45     
  Lines         5451     6868    +1417     
  Branches       454      573     +119     
===========================================
+ Hits          4484     5616    +1132     
- Misses         802     1047     +245     
- Partials       165      205      +40     

Flag	Coverage Δ
integration	68.15% <88.88%> (-1.80%)	⬇️
unittests	71.27% <55.55%> (-1.18%)	⬇️

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