Surface reactions

[RemDelaporteMathurin]

Proposed changes

This PR fixes #727 Needs to be merged after #752

Also some refactoring. Got rid of HTransportProblem.create_flux_value_fenics and merged it with define_boundary_conditions (which breaks some of the tests). I happy to not do this here and keep it for another PR.

Todo:

• [x] Write tests
• [x] Write example and usage

Types of changes

What types of changes does your code introduce to FESTIM?

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] 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

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

Further comments

@jhdark I chose not to make this a special case of the very generic class ParticleFluxBC (ie using a lambda function for value) to keep a simple implementation.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.43%. Comparing base (3fc02da) to head (e93598b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## fenicsx #753 +/- ## =========================================== + Coverage 98.39% 98.43% +0.04% =========================================== Files 28 29 +1 Lines 1554 1595 +41 =========================================== + Hits 1529 1570 +41 Misses 25 25 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[RemDelaporteMathurin]

@KulaginVladimir would you like to give this a look?