Use enum instead of symbols for indexing

[benegee]

Some containers in Trixi.jl contain symbols, e.g. node_indices in P4estInterfaceContainer. Symbols are not isbits, which could e.g. prevent GPU offloading.

[github-actions[bot]]

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 97.97980% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 96.36%. Comparing base (0046123) to head (033a817). :warning: Report is 341 commits behind head on main.

Files with missing lines	Patch %	Lines
src/solvers/dgsem_p4est/containers_3d.jl	94.12%	2 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2178   +/-   ##
=======================================
  Coverage   96.36%   96.36%           
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  Files         480      480           
  Lines       38230    38230           
=======================================
  Hits        36840    36840           
  Misses       1390     1390           

Flag	Coverage Δ
unittests	96.36% <97.98%> (ø)

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[benegee]

@sloede @ranocha @vchuravy @jmark

Introducing an enum as discussed seems to work.

[sloede]

Thanks for taking care of this! Once you're ready, please mark this PR as ready for review and request one from me.

[vchuravy]

After speaking with @maleadt in Lausanne and we decided that we might be able to permit Symbols being passed to the GPU https://github.com/JuliaGPU/GPUCompiler.jl/pull/650