Fix bug is_steady_state with TemperatureFromXDMF

[RemDelaporteMathurin]

Proposed changes

This PR fixes the bug reported on the Discourse.
This is fixed by overriding the is_steady_state method

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 ✅

Comparison is base (d8a2063) 98.91% compared to head (22ecf3e) 98.92%.
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #674   +/-   ##
=======================================
  Coverage   98.91%   98.92%           
=======================================
  Files          57       57           
  Lines        2128     2130    +2     
=======================================
+ Hits         2105     2107    +2     
  Misses         23       23           

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[jhdark]

LGTM