First-order Finite Volume Method organized with t8code

[bennibolm]

This PR adds the implementation of a first-order finite volume scheme.
The mesh is organized with t8code. Therefore, it supports MPI parallelization.

Visualization is implemented using a t8code routine directly and therefore without .h5 files and Trixi2Vtk.

TODOs:

• Rename mesh to differ from existing T8codeMesh
• Revise mesh initialization (using existing t8code routines)
• Possibility to replace messy "exchange ghost data" implementation using u_ from T8codeSolutionContainer
• Tests

[github-actions]

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Testing

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Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

Attention: 565 lines in your changes are missing coverage. Please review.

Comparison is base (7f7f058) 96.38% compared to head (824b4e2) 93.57%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
src/meshes/t8code_fv_mesh.jl	0.00%	189 Missing ⚠️
src/solvers/fv_t8code/containers.jl	0.00%	160 Missing ⚠️
src/solvers/fv_t8code/fv_2d.jl	0.00%	100 Missing ⚠️
src/callbacks_step/analysis_dg2d.jl	0.00%	55 Missing ⚠️
src/callbacks_step/stepsize_dg2d.jl	0.00%	21 Missing ⚠️
...8code_2d_fv/elixir_euler_blast_wave_hybrid_mesh.jl	0.00%	12 Missing ⚠️
src/callbacks_step/save_solution.jl	0.00%	10 Missing ⚠️
...2d_fv/elixir_euler_kelvin_helmholtz_instability.jl	0.00%	9 Missing ⚠️
src/callbacks_step/analysis.jl	0.00%	7 Missing ⚠️
src/meshes/mesh_io.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1627      +/-   ##
==========================================
- Coverage   96.38%   93.57%   -2.82%     
==========================================
  Files         455      460       +5     
  Lines       36391    36941     +550     
==========================================
- Hits        35074    34564     -510     
- Misses       1317     2377    +1060     

Flag	Coverage Δ
unittests	93.57% <0.00%> (-2.82%)	⬇️

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[bennibolm]

After T8codeMesh was implemented and merge for DG methods, I rewrote this implementation here to use the existing mesh type. Since many changes were done, I created a new PR and will close the one here.