Add velocity functions for different equations

[Arpit-Babbar]

Trixi.jl has functions to compute density and pressure for all equations, but most equations do not have those for velocity. This PR adds those functions to CompressibleEuler, MultiComponentEuler, PolytropicEuler, IdealMHD, ShallowWater equations.

[github-actions]

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Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.35%. Comparing base (0c2c367) to head (cb06499).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2086      +/-   ##
==========================================
+ Coverage   96.34%   96.35%   +0.01%     
==========================================
  Files         470      470              
  Lines       37512    37599      +87     
==========================================
+ Hits        36140    36227      +87     
  Misses       1372     1372              

Flag	Coverage Δ
unittests	96.35% <100.00%> (+0.01%)	⬆️

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[ranocha]

Thanks a lot! Could you please

• export velocity in the main file
• write a docstring describing velocity in general - maybe in equations.jl since it is a rather general interface?
• add some unit tests checking whether the correct values are returned in https://github.com/trixi-framework/Trixi.jl/blob/main/test/test_unit.jl?

[ranocha]

Thanks a lot! I just have some minor comments.

[ranocha]

Two of the GLM MHD 2D tests are failing: https://github.com/trixi-framework/Trixi.jl/actions/runs/11035734678/job/30652521365?pr=2086#step:7:1405

[andrewwinters5000]

The ShallowWaterEquations in 1D and 2D already have a velocity function, e.g., here that returns each Cartesian directions velocity. The normal direction velocity is then computed within the function if necessary.

[ranocha]

Are you fine with this, @andrewwinters5000?

[andrewwinters5000]

LGTM!