preparation for DGMulti shock capturing

[jlchan]

I'm currently adding shock capturing to DGMulti. This is a minor PR adding some interfaces and minor refactoring (to reduce the size of the full PR).

[codecov]

Codecov Report

Merging #1458 (4ee3e18) into main (f581b9d) will decrease coverage by 0.01%.
The diff coverage is 92.31%.

❗ Current head 4ee3e18 differs from pull request most recent head a326dcc. Consider uploading reports for the commit a326dcc to get more accurate results

@@            Coverage Diff             @@
##             main    #1458      +/-   ##
==========================================
- Coverage   95.72%   95.71%   -0.01%     
==========================================
  Files         358      358              
  Lines       29643    29641       -2     
==========================================
- Hits        28374    28370       -4     
- Misses       1269     1271       +2     

Flag	Coverage Δ
unittests	95.71% <92.31%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Trixi.jl	68.18% <ø> (ø)
src/solvers/dgmulti/types.jl	96.64% <0.00%> (-1.31%)	⬇️
src/callbacks_step/analysis_dgmulti.jl	80.00% <100.00%> (ø)
src/solvers/dgmulti/flux_differencing.jl	87.68% <100.00%> (-0.09%)	⬇️
src/solvers/dgmulti/flux_differencing_gauss_sbp.jl	93.16% <100.00%> (-0.05%)	⬇️

[ranocha]

Sounds like a great plan!

[ranocha]

Ok to be merged from my side if tests pass (maybe ignoring the Linux MPI CI error we don't understand...)