Improved constraints

[rozyczko]

New constraint helpers and API exposure:

• Added constrain, constrain_equal, and unconstrain functions in the new src/easyreflectometry/constraints.py module, providing user-friendly wrappers for parameter dependencies.
• Exposed these helpers in the public API via src/easyreflectometry/__init__.py and included them in __all__.

Documentation updates:

• Added a comprehensive "Constraining Parameters" section to the model.md tutorial, including examples for tying parameters, functional constraints, removing constraints, and caveats.
• Updated the assemblies library documentation to reference the new constraints tutorial section.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (e92b6b7) to head (fff5716).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           polarized_channels     #395      +/-   ##
======================================================
+ Coverage               93.69%   93.71%   +0.01%     
======================================================
  Files                      51       52       +1     
  Lines                    4519     4532      +13     
======================================================
+ Hits                     4234     4247      +13     
  Misses                    285      285              

Flag	Coverage Δ
integration	39.51% <69.23%> (+0.08%)	⬆️
unittests	93.71% <100.00%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/easyreflectometry/__init__.py	81.81% <100.00%> (+6.81%)	⬆️
src/easyreflectometry/constraints.py	100.00% <100.00%> (ø)

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