Fix for MaximumSurface and MinimumSurface

[KulaginVladimir]

Proposed changes

Thanks to @jorgensd, this PR should fix #983.

I tested the fix locally for the cases of parallel computation.

Let me know if the tests should be modified or there is a need to test the fix for mpirun via pytest-mpi.

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 99.31%. Comparing base (2c06915) to head (9b411c6).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1010   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          60       60           
  Lines        2904     2921   +17     
=======================================
+ Hits         2884     2901   +17     
  Misses         20       20           

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[RemDelaporteMathurin]

Thanks @KulaginVladimir for this fix! Just a couple of minor comments