Refactoring interfaces

[RemDelaporteMathurin]

Description

Summary

This PR refactores the interface "interface" in preparation for #1093

Related Issues

Motivation and Context

A lot of the formulation logic was in HydrogenTransportProblemDiscontinuous which was not very readable

Type of Change

• 🐛 Bug fix (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 (no functional changes, no API changes)
• 📝 Documentation update
• ✅ Test update (adding missing tests or correcting existing tests)
• 🔧 Build/CI configuration change

Testing

• All existing tests pass locally (pytest)
• I have added new tests that prove my fix is effective or that my feature works

Code Quality Checklist

• My code follows the code style of this project (Ruff formatted: ruff format .)
• My code passes linting checks (ruff check .)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

Documentation

• I have updated the documentation accordingly (if applicable)
• I have added docstrings to new functions/classes following the project conventions

Breaking Changes

Screenshots/Examples

Additional Notes

[codecov]

Codecov Report

❌ Patch coverage is 90.08264% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.92%. Comparing base (47e91b9) to head (a5e0301).
⚠️ Report is 17 commits behind head on fenicsx.

Files with missing lines	Patch %	Lines
src/festim/subdomain/interface.py	88.34%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx    #1098   +/-   ##
========================================
  Coverage    94.91%   94.92%           
========================================
  Files           44       44           
  Lines         3226     3268   +42     
========================================
+ Hits          3062     3102   +40     
- Misses         164      166    +2     

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[RemDelaporteMathurin]

@jhdark requesting your review here
The low patch coverage is due to error raises (not too interested in writing tests for these) or lines that were already untested

[RemDelaporteMathurin]

@jhdark I think this is now ready for another review