Penalty method for interfaces

[RemDelaporteMathurin]

Proposed changes

This PR implements the penalty method for interface discontinuities.

➕ Advantages

• We can set Henry vs Sieverts which we can do with the change of variable or the mixed domain approach
• Simple implementation compared to mixed domain approach

➖ Drawbacks

• requires a penalty term that could grow to infinity

We could potentially have it as a flag of the other discontinuous class method="nitsche" or method="penalty"

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

[codecov]

Codecov Report

Attention: Patch coverage is 84.31373% with 8 lines in your changes missing coverage. Please review.

Project coverage is 97.49%. Comparing base (8c5a965) to head (a81b602).
Report is 3 commits behind head on fenicsx.

Files with missing lines	Patch %	Lines
src/festim/hydrogen_transport_problem.py	84.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #918      +/-   ##
===========================================
- Coverage    97.80%   97.49%   -0.32%     
===========================================
  Files           44       44              
  Lines         2142     2193      +51     
===========================================
+ Hits          2095     2138      +43     
- Misses          47       55       +8     

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