Clear error for invalid joint-fit weights

[AndrewSazonov]

When fitting several experiments together, each experiment is given a weight that controls how strongly it influences the result. Previously, entering an invalid set of weights — a negative value, weights that are not valid numbers, or all weights set to zero — could silently spoil the fit and return meaningless results, with no indication that anything was wrong.

EasyDiffraction now checks the joint-fit weights before fitting begins and stops with a clear, explanatory message when they are negative, not-a-number, or all zero (and when the number of weights does not match the number of experiments). You get an immediate, understandable error and can fix the weights straight away, instead of chasing a confusing failed refinement.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.30%. Comparing base (681b195) to head (3ea355c).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #205   +/-   ##
========================================
  Coverage    90.29%   90.30%           
========================================
  Files          283      283           
  Lines        24152    24175   +23     
  Branches      2828     2834    +6     
========================================
+ Hits         21809    21832   +23     
  Misses        1747     1747           
  Partials       596      596           

Flag	Coverage Δ
unittests	90.30% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/easydiffraction/analysis/fitting.py	97.89% <100.00%> (+0.28%)	⬆️

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