[BUG] Fix for exports with Change of Variable method

[jhdark]

Proposed changes

Should be passing the post processing solution to the writer not the sub function

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 95.50%. Comparing base (a02ed4b) to head (1aec376).
⚠️ Report is 121 commits behind head on fenicsx.

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx    #1028      +/-   ##
===========================================
- Coverage    95.56%   95.50%   -0.07%     
===========================================
  Files           45       45              
  Lines         3089     3089              
===========================================
- Hits          2952     2950       -2     
- Misses         137      139       +2     

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