Fix surface exports

[jhdark]

Proposed changes

Needed to update some of the other surface exports to use the field that's given rather than using the field given by the user, making things easier for both the standard and discontinuous

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 94.90%. Comparing base (8fd9af4) to head (c6fee57).
⚠️ Report is 7 commits behind head on fenicsx.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx    #1058   +/-   ##
========================================
  Coverage    94.90%   94.90%           
========================================
  Files           44       44           
  Lines         3099     3102    +3     
========================================
+ Hits          2941     2944    +3     
  Misses         158      158           

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[RemDelaporteMathurin]

Thanks James, feel free to merge