Feature: DGMultiMesh from t8code forest (prototype)

[jmark]

This PR adds routines to build a DGMultiMesh from a t8code forest. It is a prototype implementation. It is not particular performant, robust and only supports conformal meshes with one element type (no hybrid meshes, no mortar interfaces).

This is a follow-up PR to https://github.com/trixi-framework/Trixi.jl/pull/2259.

[github-actions[bot]]

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Purpose and scope

• [ ] The PR has a single goal that is clear from the PR title and/or description.
• [ ] All code changes represent a single set of modifications that logically belong together.
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Code quality

• [ ] The code can be understood easily.
• [ ] Newly introduced names for variables etc. are self-descriptive and consistent with existing naming conventions.
• [ ] There are no redundancies that can be removed by simple modularization/refactoring.
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Documentation

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Testing

• [ ] The PR passes all tests.
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Performance

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Verification

• [ ] The correctness of the code was verified using appropriate tests.
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Created with :heart: by the Trixi.jl community.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 21.41280% with 356 lines in your changes missing coverage. Please review.

Project coverage is 93.46%. Comparing base (544a6f0) to head (211ced8). Report is 83 commits behind head on main.

Files with missing lines	Patch %	Lines
src/meshes/dgmulti_t8code.jl	0.00%	171 Missing :warning:
.../dgmulti_3d/elixir_euler_baroclinic_instability.jl	0.00%	99 Missing :warning:
src/meshes/mesh_io.jl	51.79%	27 Missing :warning:
src/meshes/dgmulti_meshes.jl	19.23%	21 Missing :warning:
src/callbacks_step/save_solution_dg.jl	72.00%	14 Missing :warning:
src/solvers/dgmulti/types.jl	0.00%	12 Missing :warning:
src/solvers/dgmulti/dg.jl	70.83%	7 Missing :warning:
...amples/dgmulti_3d/elixir_advection_cubed_sphere.jl	0.00%	2 Missing :warning:
...ples/dgmulti_3d/elixir_advection_prismed_sphere.jl	0.00%	1 Missing :warning:
src/callbacks_step/save_solution.jl	88.89%	1 Missing :warning:
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2270      +/-   ##
==========================================
- Coverage   96.94%   93.46%   -3.48%     
==========================================
  Files         502      506       +4     
  Lines       41682    42070     +388     
==========================================
- Hits        40405    39318    -1087     
- Misses       1277     2752    +1475     

Flag	Coverage Δ
unittests	93.46% <21.41%> (-3.48%)	:arrow_down:

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[tristanmontoya]

The CI hangs (or at least takes an extremely long time) at CI / unstructured_dgmulti - ubuntu-latest. Need to investigate why.