added the export stage in .run()

[RemDelaporteMathurin]

Proposed changes

I noticed we were not writing to VTX in run.

This is a simple fix.

Can't test it as I don't know how to read VTX file programmatically but tested it manually

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)

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (b96c636) 96.65% compared to head (4f36e55) 96.68%.

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #612      +/-   ##
===========================================
+ Coverage    96.65%   96.68%   +0.02%     
===========================================
  Files           12       12              
  Lines          359      362       +3     
===========================================
+ Hits           347      350       +3     
  Misses          12       12              

Files	Coverage Δ
festim/hydrogen_transport_problem.py	97.79% <100.00%> (+0.04%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.