Extend CompressibleEulerQuasi1D and ShallowWaterQuasi1D to DGMulti

[jlchan]

This PR adds specialized instances of functions for the non-conservative systems ShallowWaterQuasi1D and CompressibleEulerQuasi1D necessary to allow DGMulti to run.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ad384c6) 96.27% compared to head (fdb34c4) 96.27%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1797   +/-   ##
=======================================
  Coverage   96.27%   96.27%           
=======================================
  Files         438      438           
  Lines       35623    35635   +12     
=======================================
+ Hits        34294    34306   +12     
  Misses       1329     1329           

Flag	Coverage Δ
unittests	96.27% <100.00%> (+<0.01%)	⬆️

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[DanielDoehring]

Overall, this looks good to me!
Maybe you could add a comment linking/referring to

Trixi.jl/src/equations/equations.jl

Lines 109 to 120 in ad384c6

	"""
	flux(u, normal_direction::AbstractVector, equations::AbstractEquations{1})
	Enables calling `flux` with a non-integer argument `normal_direction` for one-dimensional
	equations. Returns the value of `flux(u, 1, equations)` scaled by `normal_direction[1]`.
	"""
	@inline function flux(u, normal_direction::AbstractVector,
	equations::AbstractEquations{1})
	# Call `flux` with `orientation::Int = 1` for dispatch. Note that the actual
	# `orientation` argument is ignored.
	return normal_direction[1] * flux(u, 1, equations)
	end

as the need for normal_direction might be in 1D not necessarily obvious.

In that case, we should add the same comment here:

Trixi.jl/src/equations/compressible_euler_1d.jl

Lines 411 to 414 in ad384c6

	@inline function flux_ranocha(u_ll, u_rr, normal_direction::AbstractVector,
	equations::CompressibleEulerEquations1D)
	return normal_direction[1] * flux_ranocha(u_ll, u_rr, 1, equations)
	end

[jlchan]

Thanks for the quick review, @DanielDoehring! I'll add those comments soon.