Arbitrary reaction

[Allentro]

Proposed changes

Enabled an arbitrary number of reactants and products. This resolves https://github.com/festim-dev/FESTIM/issues/720

I've enabled it so that The number of products can be 0 for radioactive decay (A -> None), but the number of reactants must be at least length 1.

Types of changes

• [ ] 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)
• [x] 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
• [ ] I have added necessary documentation (if appropriate)

[codecov[bot]]

Codecov Report

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

Project coverage is 98.39%. Comparing base (3d6b9da) to head (f862481). Report is 145 commits behind head on fenicsx.

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #730      +/-   ##
===========================================
- Coverage    98.39%   98.39%   -0.01%     
===========================================
  Files           27       28       +1     
  Lines         1437     1554     +117     
===========================================
+ Hits          1414     1529     +115     
- Misses          23       25       +2     

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