[BUG FIX] Initial condition bug

[jhdark]

Proposed changes

There was a minor bug, when using an initial condition in a multimaterial continuous hydrogen transport case, the value was interpolated over the whole domain rather than in just the volume subdomain defined. It turned out that it needs to be interpolated over cells0 rather than cells1.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.56%. Comparing base (dce543b) to head (451276f).
⚠️ Report is 104 commits behind head on fenicsx.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx    #1025   +/-   ##
========================================
  Coverage    95.56%   95.56%           
========================================
  Files           45       45           
  Lines         3089     3089           
========================================
  Hits          2952     2952           
  Misses         137      137           

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