set! for PrescribedAtmosphere

[glwagner]

It would be fun to support set! for PrescribedAtmosphere, which would give us more flexibility in constructing idealized atmospheric states like #251 .

Oceananigans CliMA/Oceananigans.jl#3929 is related, since having set! for FieldTimeSeries will help a lot.

I'm envisioning something like

# Atmospheric state parameters
Tₐ = 273.15 + 20 # Kelvin
u₁₀ = 10 # wind at 10 m, m/s
qₐ = 0.01 # specific humidity
Qℓw = 400 # shortwave radiation (W m⁻², positive means heating right now)

# Build the atmosphere
atmosphere_grid = RectilinearGrid(size=(), topology=(Flat, Flat, Flat))
atmosphere_times = range(0, 1day, length=3)
atmosphere = PrescribedAtmosphere(atmosphere_grid, atmosphere_times)

set!(atmosphere, T=Tₐ, u=u₁₀, q=qₐ, Qℓw=Qℓw)