Fix warnings + remove warnings in test

[RemDelaporteMathurin]

Proposed changes

This is the fix for #740

Also took this opportunity to reduce the number of warnings in the test suite.

Types of changes

What types of changes does your code introduce to FESTIM?

• [x] 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

• [x] Black formatted
• [x] 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)

[codecov[bot]]

Codecov Report

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

Project coverage is 99.52%. Comparing base (315aebd) to head (f30ba73). Report is 12 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #773 +/- ## ========================================== + Coverage 99.51% 99.52% +0.01% ========================================== Files 58 58 Lines 2476 2533 +57 ========================================== + Hits 2464 2521 +57 Misses 12 12 ```

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