Snow submodel notions

[tyralla]

This is a subissue of #188. It serves to discuss and document the unification of the parameter and sequence names of the main models' snow modules (listed in #188), as well as the parameter and sequence names of snow_cn and snow_cn_minmax.

I will continuously edit the following tables.

control parameters

(some subarea parameters handle relative and other handle absolute values)

HBV	LARSIM	WHMod	WALRUS	Snow old	Snow new
NmbZones	NHRU	NmbZones	NU	-	NmbHRU
ZoneArea	FT	ZoneArea	AUR	-	HRUArea
CFMax	GTF	DegreeDayFactor	DDF	CN2	DegreeDayFactor

state sequences

HBV	LARSIM	WHMod	WALRUS	Snow old	Snow new
-	WAeS	SnowPack	SP	G	SnowPack
SP	WATS	-	-	-	LiquidWaterContent
WC	-	-	-	-	-

[tyralla]

I think we should follow HydPy-Evap's naming style whenever appropriate. Many names are reusable (NmbHRU, HRUArea, etc.).