Read in capacity for parallel TreeMesh

[benegee]

Fixes restarting with AMR and MPI.
Resolves #1912

[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 (b362795).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1913   +/-   ##
=======================================
  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 get a CI test for this, please?

[sloede]

Nice work! As commented by @ranocha, would it be possible to add a test for this as well?