Upgrade baroclinic instability elixir

[efaulhaber]

This PR

• replaces the perturbation in the baroclinic instability elixir (which is the one used in the original paper that suggested this test for shallow atmosphere models) by an improved version that was suggested in the paper where this elixir is based on,
• renames some parameters to be more self-explanatory,
• implements a steady-state RHS correction, as described in Correct RHS by subtracting RHS of a steady state solution #980 (also closes Correct RHS by subtracting RHS of a steady state solution #980 and [Proof of Concept] Correct RHS by subtracting RHS of a steady state solution #979),
• imports Downloads:download in every elixir that was using the deprecated download function,
• removes using Downloads:download in every elixir that doesn't use this function.

The last two are entirely unrelated to the baroclinic instability, but I just happened to notice this, and I didn't want to create a new PR for this.

[efaulhaber]

There's something wrong. The results after 10 days look a bit different.

[ranocha]

Please request my review again when you think everything is fine

[codecov]

Codecov Report

Merging #981 (13777b2) into main (edcbadb) will increase coverage by 5.86%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #981      +/-   ##
==========================================
+ Coverage   87.80%   93.66%   +5.86%     
==========================================
  Files         287      287              
  Lines       20941    20972      +31     
==========================================
+ Hits        18387    19643    +1256     
+ Misses       2554     1329    -1225     

Flag	Coverage Δ
unittests	93.66% <100.00%> (+5.86%)	⬆️

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

Impacted Files	Coverage Δ
...ples/p4est_2d_dgsem/elixir_euler_blast_wave_amr.jl	100.00% <ø> (ø)
examples/p4est_2d_dgsem/elixir_euler_sedov.jl	92.86% <ø> (ø)
..._source_terms_nonconforming_unstructured_curved.jl	100.00% <ø> (ø)
examples/tree_2d_dgsem/elixir_shallowwater_ec.jl	100.00% <ø> (ø)
...tree_2d_dgsem/elixir_shallowwater_well_balanced.jl	100.00% <ø> (ø)
...amples/unstructured_2d_dgsem/elixir_euler_sedov.jl	92.86% <ø> (ø)
...st_3d_dgsem/elixir_euler_baroclinic_instability.jl	98.97% <100.00%> (+0.28%)	⬆️
examples/p4est_3d_dgsem/elixir_euler_sedov.jl	93.75% <100.00%> (ø)
src/auxiliary/auxiliary.jl	90.70% <0.00%> (+0.70%)	⬆️
src/callbacks_step/amr_dg3d.jl	98.37% <0.00%> (+0.81%)	⬆️
... and 35 more

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[efaulhaber]

Alright, it works now. It was just a sign error.

[efaulhaber]

CI finished over 1 hour ago. I don't think codecov will be reporting anything.