Feature: Checkpointing for T8codeMesh

[jmark]

This PR adds checkpointing for T8codeMesh. By this, routines like save_mesh and load_mesh are supported.

[github-actions[bot]]

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Code quality

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Testing

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Performance

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 73.18182% with 59 lines in your changes missing coverage. Please review.

Project coverage is 88.99%. Comparing base (58acd35) to head (6396931).

Files	Patch %	Lines
src/meshes/t8code_mesh.jl	75.64%	38 Missing :warning:
src/meshes/mesh_io.jl	68.97%	18 Missing :warning:
src/callbacks_step/save_restart_dg.jl	0.00%	3 Missing :warning:

:exclamation: There is a different number of reports uploaded between BASE (58acd35) and HEAD (6396931). Click for more details.

HEAD has 3 uploads less than BASE

| Flag | BASE (58acd35) | HEAD (6396931) | |------|------|------| |unittests|26|23|

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1980 +/- ## ========================================== - Coverage 96.19% 88.99% -7.20% ========================================== Files 460 460 Lines 36993 37157 +164 ========================================== - Hits 35584 33065 -2519 - Misses 1409 4092 +2683 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1980/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/1980/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `88.99% <73.18%> (-7.20%)` | :arrow_down: | 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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