DGMulti shock capturing part 1: cleanup

[jlchan]

This splits off "cleanup" aspects of #2872 into a smaller PR.

This PR does not implement any new functionality, just cleans up existing functionality for GaussSBP solvers and adds a test in 3D (which was supported but not tested).

[github-actions]

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Created with ❤️ by the Trixi.jl community.

[DanielDoehring]

Thanks Jesse! This is much easier to digest. I have only some very minor comments regarding formatting and naming.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.11%. Comparing base (3805c7c) to head (9349737).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2940      +/-   ##
==========================================
+ Coverage   97.10%   97.11%   +0.01%     
==========================================
  Files         622      622              
  Lines       48228    48235       +7     
==========================================
+ Hits        46831    46843      +12     
+ Misses       1397     1392       -5     

Flag	Coverage Δ
unittests	97.11% <100.00%> (+0.01%)	⬆️

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