Gradient layer

[andped10]

Added gradient layer

Makes a discrete stack of layers where the sld and isld are a lineary interpolation between the initial and final material types.

The thickness of the gradient layer is given by the thickness of layer 0 multiplied by the number of layers

• Constraints are use to ensure that all layers in the stack is the same as layer 0

MultiLayer

• changed to be possible to pass type to constructor
• only one argument per line

pyproject.toml

• added isort to ensure our imports are sorted and that there only is one import per line.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e759eff) 97.86% compared to head (d5ec71a) 97.90%.
Report is 17 commits behind head on develop.

❗ Current head d5ec71a differs from pull request most recent head aa35481. Consider uploading reports for the commit aa35481 to get more accurate results

Files	Patch %	Lines
tests/sample/items/test_gradient_layer.py	97.54%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #71      +/-   ##
===========================================
+ Coverage    97.86%   97.90%   +0.03%     
===========================================
  Files           41       43       +2     
  Lines         3050     3248     +198     
===========================================
+ Hits          2985     3180     +195     
- Misses          65       68       +3     

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[rozyczko]

Very well written code and exhaustive unit tests.
Minor, optional, issues raised.