correct units for scattering lengths #377

[rozyczko]

Unit and Dependency Handling Improvements:

• The unit metadata for MaterialDensity has been corrected: scattering lengths (scattering_length_real, scattering_length_imag) now use angstrom, and SLDs (sld, isld) use 1/angstrom^2, matching physical expectations and fixing inconsistencies in both code and material string representations.
• The dependency expression for SLD calculation has been refactored to use a physically meaningful formula with units, and Avogadro's constant is now an explicit parameter. The SLD dependency wiring is now centralized in a helper method, ensuring derived parameters always have the correct unit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (e7e3693) to head (7cd440b).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #419   +/-   ##
========================================
  Coverage    94.57%   94.57%           
========================================
  Files           54       54           
  Lines         5515     5516    +1     
========================================
+ Hits          5216     5217    +1     
  Misses         299      299           

Flag	Coverage Δ
integration	38.37% <30.00%> (+0.04%)	⬆️
unittests	94.57% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...etry/sample/elements/materials/material_density.py	99.13% <100.00%> (+<0.01%)	⬆️

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