Implement a buoyancy frequency (squared) functionality that is "cloudy-region-aware"

[glwagner]

Once CliMA/Oceananigans.jl#4967 lands in Oceananigans, we will have the ability to implement a buoyancy abstraction in the AtmosphereModel for computing the vertical derivative of buoyancy (or perhaps more precisely, the buoyancy frequency squared) that extends buoyancy_force and buoyancy_tracers.

This opens the possibility to computing the buoyancy frequency in a way that is aware of saturation, eg equations 18-19 in this paper:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018MS001446

This is done by extending ∂z_b defined here:

https://github.com/CliMA/Oceananigans.jl/blob/d3f0df86233828fb7c93b67591690c77102c3479/src/BuoyancyFormulations/buoyancy_force.jl#L94

which takes in buoyancy_force(model) and buoyancy_tracers(model) as key arguments.

[glwagner]

https://www.atmos.washington.edu/~durrand/pdfs/AMS/1982a_Durran_Klemp_JAS.pdf also useful