ConvectiveHeating implemented

[milankl]

closes #567

[milankl]

@KristianJS to be used like

spectral_grid = SpectralGrid()
convection = ConvectiveHeating(spectral_grid, p₀ = 525) # specify non-defaults as keyword args here     
model = PrimitiveDryModel(spectral_grid; convection)    # pass on to model constructor
simulation = initialize!(model)
run!(simulation)

ConvectiveHeating is implemented as a parameterization (not a forcing) given it's formulated in pressure coordinates and especially because it's presumably not to be used in combination with other convection schemes. So passing convection here on will use that instead of the more complicated default Betts-Miller scheme. Not tested yet, would you?