Save and load user_data for p4est

[benegee]

This fixes restating when using AMR.
Resolves #1914

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.30%. Comparing base (dd05156) to head (f0403af).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1915   +/-   ##
=======================================
  Coverage   96.30%   96.30%           
=======================================
  Files         450      450           
  Lines       36182    36182           
=======================================
  Hits        34843    34843           
  Misses       1339     1339           

Flag	Coverage Δ
unittests	96.30% <100.00%> (ø)

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[ranocha]

Can we have a CI test for this?

[sloede]

Looks more or less good to me, I only have one small suggestion.

I also second @ranocha's request for a test (if possible).

[sloede]

I know there wasn't a comment here before, but maybe you can add one that explains what this line does?

[benegee]

I am still unhappy with this fix. While new_p4est allows to just allocate user data, load_p4est requires to also provide the data. So in the end I am only saving the data to have memory allocated upon restart.

I also do not know if there was a good reason for not saving user data in the first place.