temperature-based "snow tower" prevention mechanism for `lland_knauf`?

[tyralla]

Such an approach will soon be in master (see #174), but only for lland_dd. Principally, it would not require too much work to apply the same method to lland_knauf (and lland_knauf_ic). However, the snow module of lland_knauf relies on many more meteorological input factors than just temperature, so we should first think about whether it would be okay to modify only one of these properties and in what equations TKor should be replaced with ATKor.