Initial conditions in mixed-domain cases

[jhdark]

Description

Summary

Related Issues

Motivation and Context

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 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.77%. Comparing base (1ec8975) to head (94d4bd7).
⚠️ Report is 7 commits behind head on fenicsx.

Files with missing lines	Patch %	Lines
src/festim/hydrogen_transport_problem.py	66.66%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx    #1066      +/-   ##
===========================================
- Coverage    94.82%   94.77%   -0.06%     
===========================================
  Files           44       44              
  Lines         3132     3136       +4     
===========================================
+ Hits          2970     2972       +2     
- Misses         162      164       +2     

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[RemDelaporteMathurin]

I reviewed this with @jhdark yesterday, also tested it locally, all good to go, just needs a test/example demonstrating the capability