GLM speed calculations for coupled semidiscretizations

[SimonCan]

The callback that computes the glm speed was only working for single semidiscretizations. With this change we can now use it for coupled semidiscretizations with arbitrarily many GLM-MHD systems and the user can specify which semidiscretization needs to get its glm speed calculated.

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 89.18919% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 96.10%. Comparing base (adba1bb) to head (d195d97).

Files	Patch %	Lines
src/callbacks_step/glm_speed.jl	81.82%	2 Missing ⚠️
...c/semidiscretization/semidiscretization_coupled.jl	90.91%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1835      +/-   ##
==========================================
+ Coverage   96.08%   96.10%   +0.01%     
==========================================
  Files         450      451       +1     
  Lines       36215    36241      +26     
==========================================
+ Hits        34797    34826      +29     
+ Misses       1418     1415       -3     

Flag	Coverage Δ
unittests	96.10% <89.19%> (+0.01%)	⬆️

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[andrewwinters5000]

Why is another version of the flux_nonconservative_powell needed for the coupling procedure?

[sloede]

Thanks, this looks almost ready. I noticed a few small things, but afterwards this is good to go IMHO

[sloede]

Final note, then ready to merge 🙂

[sloede]

LGTM, thank you for bearing with me!