Bilayer support

[rozyczko]

As requested and described by @jfkcooper this is the library implementation of bilayers in EasyReflectometryLib.

Please scrutinize docs\tutorials\simulation\bilayer.ipynb for correctness and API.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 91.47287% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.30%. Comparing base (205db70) to head (fac389f).

Files with missing lines	Patch %	Lines
src/easyreflectometry/sample/assemblies/bilayer.py	91.40%	0 Missing and 11 partials ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           append_sample     #292      +/-   ##
=================================================
+ Coverage          88.10%   88.30%   +0.20%     
=================================================
  Files                 45       46       +1     
  Lines               2471     2600     +129     
  Branches             288      307      +19     
=================================================
+ Hits                2177     2296     +119     
+ Misses               233      232       -1     
- Partials              61       72      +11     

Flag	Coverage Δ
unittests	88.30% <91.47%> (+0.20%)	⬆️

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Files with missing lines	Coverage Δ
src/easyreflectometry/sample/__init__.py	100.00% <100.00%> (ø)
src/easyreflectometry/sample/assemblies/bilayer.py	91.40% <91.40%> (ø)

... and 1 file with indirect coverage changes

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