Enable SLD dependence on material data

[rozyczko]

This pull request introduces a new feature to the MaterialDensity class, allowing the SLD (scattering length density) and ISLD parameters to be decoupled from the chemical formula and density, making them directly editable or fittable. The changes also ensure that serialization and deserialization preserve this decoupling state and the associated manual values.

MaterialDensity decoupling and serialization improvements:

• Added the sld_coupled property and setter to allow toggling whether sld/isld are derived from formula/density or are independent, directly settable parameters. Setting this property updates dependencies and preserves or discards manual values as appropriate.
• Extended the serialization logic to persist the sld_coupled state and manual sld/isld values, ensuring that decoupled/manual values are not lost on save/load. Backward compatibility is maintained for older project files.
• Updated the chemical_structure setter to also update the molecular weight when the formula changes, preventing mismatches in derived SLD values.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.43%. Comparing base (8b13521) to head (0fa1340).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #403      +/-   ##
===========================================
+ Coverage    94.40%   94.43%   +0.03%     
===========================================
  Files           54       54              
  Lines         5500     5532      +32     
===========================================
+ Hits          5192     5224      +32     
  Misses         308      308              

Flag	Coverage Δ
integration	38.53% <14.70%> (-0.14%)	⬇️
unittests	94.43% <100.00%> (+0.03%)	⬆️

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Files with missing lines	Coverage Δ
...etry/sample/elements/materials/material_density.py	99.11% <100.00%> (+0.34%)	⬆️

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