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 98.77%. Comparing base (77c97b3) to head (ad3b10b).

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #753      +/-   ##
===========================================
- Coverage    98.92%   98.77%   -0.16%     
===========================================
  Files           36       37       +1     
  Lines         1584     1630      +46     
===========================================
+ Hits          1567     1610      +43     
- Misses          17       20       +3     

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[RemDelaporteMathurin]

@KulaginVladimir would you like to give this a look?