Allow for computing multiple fluxes

[charleskawczynski]

ClimaAtmos calls RRTMGP in this way:

function update_sw_fluxes!(::AllSkyRadiationWithClearSkyDiagnostics, model)
    RRTMGP.RTESolver.solve_sw!(
        model.solver,
        model.max_threads,
        model.lookups.lookup_sw,
    )
    parent(model.face_clear_sw_flux_up) .= parent(model.face_sw_flux_up)
    parent(model.face_clear_sw_flux_dn) .= parent(model.face_sw_flux_dn)
    parent(model.face_clear_sw_direct_flux_dn) .=
        parent(model.face_sw_direct_flux_dn)
    parent(model.face_clear_sw_flux) .= parent(model.face_sw_flux)
    RRTMGP.RTESolver.solve_sw!(
        model.solver,
        model.max_threads,
        model.lookups.lookup_sw,
        model.lookups.lookup_sw_cld,
    )
end

So, we're doing two passes over solve_sw!, which is likely not ideal. The cost of the callback compared to step! is staggering:

Despite calling this infrequently, it's still probably worth figuring out a way to do this in a single pass (since some variables may be shareable between the two passes).