Fix t8code ocassionally segfaults

[jmark]

Especially in GitHub's CI the pipeline randomly fails due to segfaults caused by T8codeMesh. This is due to the undeterministic order of clean-up by Julia's GC. t8code allocates several MPI objects which have to be cleaned up before the MPI runtime shuts down. This PR provides a workaround by explicitly finalizing the T8codeMesh objects in each elixir. I observed that this prevents the segfaults.

Any input for a better, more elegant solution is welcome.

This PR closes #1845.

[github-actions]

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[JoshuaLampert]

Thanks! This closes #1845, right?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.30%. Comparing base (e6ba838) to head (759475d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2043   +/-   ##
=======================================
  Coverage   96.30%   96.30%           
=======================================
  Files         467      467           
  Lines       37263    37265    +2     
=======================================
+ Hits        35886    35888    +2     
  Misses       1377     1377           

Flag	Coverage Δ
unittests	96.30% <100.00%> (+<0.01%)	⬆️

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[DanielDoehring]

Thanks! This closes #1845, right?

Maybe then @patrickersing can take a look?

[patrickersing]

Thanks! This closes #1845, right?

Maybe then @patrickersing can take a look?

I would be happy to help, but I have no experience with t8code or MPI, so it would be good if someone who is more familiar with this does a review.

[ranocha]

Thanks! I just have a question - if it's fine, this can be merged afterward