fixing #370

[rozyczko]

Enhancements to error handling and user feedback:

• The mcmc_sample function now raises a ValueError if all data points have zero variance (except when using the explicit 'mighell' objective), preventing undefined likelihoods. It also issues detailed warnings when the Mighell transform is applied, either to all points or to a subset with zero variance, clarifying the implications for the user.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.61%. Comparing base (5cb7b9d) to head (4dabce3).

Files with missing lines	Patch %	Lines
src/easyreflectometry/fitting.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           core_fixes     #388      +/-   ##
==============================================
- Coverage       31.66%   31.61%   -0.05%     
==============================================
  Files              48       48              
  Lines            3840     3846       +6     
==============================================
  Hits             1216     1216              
- Misses           2624     2630       +6     

Flag	Coverage Δ
integration	31.61% <0.00%> (-0.05%)	⬇️
unittests	0.00% <0.00%> (ø)

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Files with missing lines	Coverage Δ
src/easyreflectometry/fitting.py	16.88% <0.00%> (-0.46%)	⬇️

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