Dirichlet boundary conditions

[RemDelaporteMathurin]

Proposed changes

This is a second attempt at #602

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (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
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

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

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

Attention: 44 lines in your changes are missing coverage. Please review.

Comparison is base (2fc987a) 95.06% compared to head (7c9f710) 84.14%.

Additional details and impacted files

@@             Coverage Diff              @@
##           fenicsx     #606       +/-   ##
============================================
- Coverage    95.06%   84.14%   -10.92%     
============================================
  Files            9       11        +2     
  Lines          223      347      +124     
============================================
+ Hits           212      292       +80     
- Misses          11       55       +44     

Files	Coverage Δ
festim/__init__.py	69.56% <100.00%> (+2.89%)	⬆️
festim/hydrogen_transport_problem.py	97.63% <100.00%> (+0.22%)	⬆️
festim/boundary_conditions/dirichlet_bc.py	75.00% <75.00%> (ø)
festim/boundary_conditions/sieverts_bc.py	65.71% <65.71%> (ø)
festim/helpers.py	42.85% <28.57%> (-57.15%)	⬇️

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