Allows tolerances to be callable

[kaelyndunnell]

Proposed changes

Allows tolerances to be callable (not just an integer or float). Solves callable and returns a float before passing tolerance to fenicsx.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 96.15%. Comparing base (01ab7f9) to head (ceae144).
Report is 290 commits behind head on fenicsx.

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #958      +/-   ##
===========================================
+ Coverage    96.02%   96.15%   +0.13%     
===========================================
  Files           45       46       +1     
  Lines         2490     2575      +85     
===========================================
+ Hits          2391     2476      +85     
  Misses          99       99              

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[RemDelaporteMathurin]

Thanks for this Kaelyn! Just a couple of suggestions to make this better