Add subcell positivity limiting of non-linear variables

[bennibolm]

This PR expand the support of the subcell positivity limiting to non-linear variables. This allows for instance to reduce the initial pressure from 1.0 to 0.01 in the existing mhd elixir examples/tree_2d_dgsem/elixir_mhd_shockcapturing_subcell.jl.

The limiting is done as follows (same as in Fluxo):
Now, and as for the Zalesak-type limiting before, each node is looked at separately. Then, we look at the antidiffusive_flux (DG flux - FV flux) for all 4 (=2*dimension) directions separately again.
So, we want to calculate the maximum amount of flux, we can add to meet our given bounds.
For this, we start with the current (from local limiting and positivity limiting for conservative variables) coefficient and increase it until the resulting solution fulfills the bounds. For this iterative approach, we use a Newton-bisection method.

[github-actions]

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[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (c7cee98) 96.38% compared to head (b16d742) 96.38%.

Files	Patch %	Lines
src/solvers/dgsem_tree/subcell_limiters_2d.jl	95.18%	4 Missing ⚠️
src/callbacks_stage/subcell_bounds_check_2d.jl	80.00%	3 Missing ⚠️
src/callbacks_stage/subcell_bounds_check.jl	71.43%	2 Missing ⚠️
src/equations/compressible_euler_2d.jl	90.91%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1738    +/-   ##
========================================
  Coverage   96.38%   96.38%            
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  Files         454      455     +1     
  Lines       36249    36391   +142     
========================================
+ Hits        34937    35074   +137     
- Misses       1312     1317     +5     

Flag	Coverage Δ
unittests	96.38% <93.42%> (+<0.01%)	⬆️

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[amrueda]

Looks great to me!!
Just a couple minor comments here.

[bennibolm]

Thanks @amrueda 🙌

[sloede]

Not much left to do imho, ready to party by the end of the week 👯‍♂️

[sloede]

LGTM, thanks! There are just two names that I'd like to get @ranocha's feedback on; if he agrees, this can be merged

[sloede]

LGTM, thanks!