Fix bug with XDMF export in mode last

[RemDelaporteMathurin]

Proposed changes

This is the fix for #738

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 99.50%. Comparing base (f1f9a71) to head (86d54ce).
Report is 156 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #796      +/-   ##
==========================================
- Coverage   99.53%   99.50%   -0.03%     
==========================================
  Files          59       59              
  Lines        2581     2647      +66     
==========================================
+ Hits         2569     2634      +65     
- Misses         12       13       +1     

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[KulaginVladimir]

@RemDelaporteMathurin, fix works fine. Just a comment on the test description.