Make prognostics available for forcing, drag in Barotropic/SWM

[milankl]

With this PR the drag formulation from #394 can be simplified to

function SpeedyWeather.drag!(   diagn::SpeedyWeather.DiagnosticVariablesLayer,
                                progn::SpeedyWeather.PrognosticVariablesLayer,
                                drag::JetDrag,
                                time::DateTime,
                                model::SpeedyWeather.ModelSetup)

    (;vor) = progn
    (;vor_tend) = diagn.tendencies
    (;ζ₀) = drag
    
    (;radius) = model.spectral_grid
    r = radius/drag.time_scale.value
    for lm in eachindex(vor,vor_tend,ζ₀)
        vor_tend[lm] -= r*(vor[lm] - ζ₀[lm])
    end
    
    return nothing
end

because PrognosticVariablesLayer is passed on