Read in capacity for parallel TreeMesh

[benegee]

Fixes restarting with AMR and MPI. Resolves #1912

[github-actions[bot]]

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Testing

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Created with :heart: by the Trixi.jl community.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.11%. Comparing base (76719a8) to head (9f27450).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1913 +/- ## ======================================= Coverage 96.11% 96.11% ======================================= Files 460 460 Lines 36926 36926 ======================================= Hits 35490 35490 Misses 1436 1436 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1913/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1913/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `96.11% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ranocha]

Can we get a CI test for this, please?