Prep `DGMulti` for non-conservative shock capturing

[jlchan]

Should also improve efficiency and extend non-conservative terms to curved meshes.

https://github.com/trixi-framework/Trixi.jl/pull/1478 should be merged first to reduce the size of this PR.

[codecov[bot]]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (c3c0986) 89.67% compared to head (aa0a995) 86.59%. Report is 1 commits behind head on main.

Files	Patch %	Lines
src/solvers/dgmulti/flux_differencing.jl	0.00%	22 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1479      +/-   ##
==========================================
- Coverage   89.67%   86.59%   -3.08%     
==========================================
  Files         458      458              
  Lines       36400    36374      -26     
==========================================
- Hits        32640    31496    -1144     
- Misses       3760     4878    +1118     

Flag	Coverage Δ
unittests	86.59% <0.00%> (-3.08%)	:arrow_down:

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[ranocha]

Shall this be merged before the next breaking release?

[jlchan]

I probably won't have time to finish this before then, so I'm happy to wait until the next release.