Feature: Mesh-IO and Save Solution support for DGMulti

[jmark]

Title says it all. It is still considered draft. Any comments, advice and contributions are welcome!

The pendant Trixi2Vtk PR: trixi-framework/Trixi2Vtk.jl#103

[github-actions]

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Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

Attention: Patch coverage is 62.04380% with 52 lines in your changes missing coverage. Please review.

Project coverage is 96.31%. Comparing base (24107f4) to head (a3e0f55).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/meshes/mesh_io.jl	59.65%	23 Missing ⚠️
src/meshes/dgmulti_meshes.jl	22.22%	21 Missing ⚠️
src/callbacks_step/save_solution_dg.jl	84.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2259      +/-   ##
==========================================
- Coverage   96.42%   96.31%   -0.11%     
==========================================
  Files         490      490              
  Lines       39426    39556     +130     
==========================================
+ Hits        38015    38095      +80     
- Misses       1411     1461      +50     

Flag	Coverage Δ
unittests	96.31% <62.04%> (-0.11%)	⬇️

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