new OutflowBC

[jhdark]

Description

Summary

When including an advection term in a FESTIM simulation, an additional term may be required from integration by parts: + (v·n) u w ds.

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 82.35294% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.46%. Comparing base (d7d8372) to head (c2efd18).
⚠️ Report is 4 commits behind head on fenicsx.

Files with missing lines	Patch %	Lines
src/festim/boundary_conditions/outflow_bc.py	70.17%	17 Missing ⚠️
src/festim/exports/advection_flux.py	95.65%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx    #1081      +/-   ##
===========================================
- Coverage    94.77%   94.46%   -0.31%     
===========================================
  Files           44       46       +2     
  Lines         3174     3269      +95     
===========================================
+ Hits          3008     3088      +80     
- Misses         166      181      +15     

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