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?

• [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
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] 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 99.50%. Comparing base (f1f9a71) to head (86d54ce). Report is 156 commits behind head on main.

Additional details and impacted files

```diff @@ 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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