Henry solubillity with traps

[RemDelaporteMathurin]

Proposed changes

Fixes #454 by adding a NonImplementedError when chemical potential with Henry law is set with traps.
The solver doesn't converge when using c = theta ** 2 * S for some reason...

Another good argument for using a discontinuity instead of this variable change!!!

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)