fixed issue #367

[rozyczko]

This pull request standardizes the handling of resolution function width conventions across the codebase, ensuring that all resolution functions return sigma (the standard deviation of a Gaussian) and that each engine wrapper (refnx and refl1d) interprets these values correctly. This fixes long-standing inconsistencies (see GitHub issue #367) where the same resolution function could produce different results depending on the backend. The PR also adds comprehensive cross-engine regression tests and updates all affected tests to the new conventions.

Resolution function standardization:

• All resolution function classes now return sigma (standard deviation) from their smearing() method, regardless of their parameterization (e.g., FWHM percentage, FWHM knots, or pointwise sigma). A new constant, SIGMA_TO_FWHM, is used for conversion where needed.
• The docstrings and comments have been updated throughout to clarify the sigma/FWHM conventions and responsibilities of each wrapper.

Engine wrapper updates:

• The refnx and refl1d wrappers now correctly convert sigma to the expected width convention for their respective engines: refnx expects FWHM, refl1d expects sigma. This removes previous ad-hoc or missing conversions.

Testing improvements:

• A new integration test (test_cross_engine_resolution.py) verifies that the same model and resolution function produce consistent results across both engines, with a tolerance wide enough to catch convention errors but not minor algorithmic differences.

• All affected unit tests have been updated to expect sigma output from smearing(), using the new SIGMA_TO_FWHM constant for assertions.

Other:

• Removed unused imports and cleaned up code for clarity.

This change ensures that resolution smearing is handled consistently and transparently, preventing subtle bugs and making cross-engine comparisons reliable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.65%. Comparing base (a472411) to head (2ae07f5).

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #382       +/-   ##
============================================
+ Coverage     0.00%   31.65%   +31.65%     
============================================
  Files           42       48        +6     
  Lines         3578     3832      +254     
============================================
+ Hits             0     1213     +1213     
+ Misses        3578     2619      -959     

Flag	Coverage Δ
integration	31.65% <100.00%> (+31.65%)	⬆️
unittests	0.00% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rc/easyreflectometry/calculators/refl1d/wrapper.py	63.72% <ø> (ø)
src/easyreflectometry/calculators/refnx/wrapper.py	80.64% <100.00%> (ø)
...rc/easyreflectometry/model/resolution_functions.py	80.39% <100.00%> (+80.39%)	⬆️

... and 40 files with indirect coverage changes

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