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?

• 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)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.52%. Comparing base (315aebd) to head (f30ba73).
Report is 225 commits behind head on main.

Additional details and impacted files

@@            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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