Finish documenting the "snow tower" prevention mechanism of lland_dd #174

I think the documentation of this new feature is a little sparse, but generally okay. However, we should cite at least the relevant literature (I do not know who published this approach first). Further hints on what is missing or could be improved are welcome, of course.

The relevant sections should soon be online:

• https://hydpy-dev.github.io/hydpy/master/lland_dd.html#snow-towers
• https://hydpy-dev.github.io/hydpy/master/lland.html#hydpy.models.lland.lland_model.Calc_ATKor_V1
• https://hydpy-dev.github.io/hydpy/master/lland.html#hydpy.models.lland.lland_model.Calc_AWGTF_V1
• https://hydpy-dev.github.io/hydpy/master/lland.html#hydpy.models.lland.lland_model.Calc_AWNied_V1