Fix to allow D as Function in discontinuous case

[RemDelaporteMathurin]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.91%. Comparing base (32d138d) to head (5503695).
⚠️ Report is 76 commits behind head on fenicsx.

Files with missing lines	Patch %	Lines
src/festim/hydrogen_transport_problem.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx    #1040      +/-   ##
===========================================
- Coverage    94.95%   94.91%   -0.05%     
===========================================
  Files           44       44              
  Lines         3112     3107       -5     
===========================================
- Hits          2955     2949       -6     
- Misses         157      158       +1     

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