Add Held-Suarez test case

[benegee]

Implemented following Souza et al..

[benegee]

@MarcoArtiano Maaybe some things still need to be adapted to your equations.

[MarcoArtiano]

@MarcoArtiano Maaybe some things still need to be adapted to your equations.

Thanks! I think the source term might need an adjustment. I'll take a look at that.

[MarcoArtiano]

Thanks Benedict! I've added a few comments/suggestions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.30%. Comparing base (4119575) to head (3bf0b0a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #122   +/-   ##
=======================================
  Coverage   97.30%   97.30%           
=======================================
  Files          30       30           
  Lines        3903     3903           
=======================================
  Hits         3798     3798           
  Misses        105      105           

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[benegee]

I addressed the remaining points, @MarcoArtiano.

I particular, I corrected the source terms as discussed.
I also added

TrixiAtmo.jl/test/test_3d_potential_temperature.jl

Lines 177 to 182 in 0d3429a

	import ..CI_ON_MACOS
	if CI_ON_MACOS
	global _rtol = 7e-8 # increased error tolerance
	else
	global _rtol = sqrt(eps(Float64)) # default
	end

to select a higher tolerance, but only when running on macOS. Something like this could be helpful in other PRs as well, I think.

[MarcoArtiano]

Thanks Benedict! I just left a minor comment.