Allow D to be func for multi vol subdomains

[kaelyndunnell]

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)

[RemDelaporteMathurin]

I don't think this is needed, it's been fixed in #1040

[kaelyndunnell]

Lol RIP.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.92%. Comparing base (f4f7875) to head (9a95372).

Files with missing lines	Patch %	Lines
src/festim/hydrogen_transport_problem.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx    #1041      +/-   ##
===========================================
- Coverage    94.94%   94.92%   -0.02%     
===========================================
  Files           44       44              
  Lines         3124     3132       +8     
===========================================
+ Hits          2966     2973       +7     
- Misses         158      159       +1     

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