Subdomain temperature fields

[jhdark]

Proposed changes

Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request. If it fixes a bug or resolves a feature request, be sure to link to that issue.

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

❌ Patch coverage is 82.53968% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.03%. Comparing base (e9788b8) to head (0a0422d).

Files with missing lines	Patch %	Lines
src/festim/hydrogen_transport_problem.py	80.70%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx    #1060      +/-   ##
===========================================
- Coverage    94.90%   93.03%   -1.87%     
===========================================
  Files           44       44              
  Lines         3102     3132      +30     
===========================================
- Hits          2944     2914      -30     
- Misses         158      218      +60     

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