Add nfactors for dampening wintertime air temperature forcings

Additional breaking change:

Since nfactors don't really work with Kelvin (at least not nicely), and I've quite frankly gotten sick of Kelvin causing problems, all temperatures are now handled in Celsius.

This may cause some problems with Unitful in the future, but it's used sparingly at the moment, so it doesn't really matter.

docs/src/manual/overview.md

@@ -46,7 +46,7 @@ Notice that, in the example above, it is types such as `Soil`, `Heat`, `SFCC`, e
 """ Variable definitions for heat conduction (enthalpy) on soil layer. """
 variables(soil::Soil, heat::Heat{:H}) = (
     Prognostic(:H, Float"J/m^3", OnGrid(Cells)),
-    Diagnostic(:T, Float"K", OnGrid(Cells)),
+    Diagnostic(:T, Float"°C", OnGrid(Cells)),
     Diagnostic(:C, Float"J//K*/m^3", OnGrid(Cells)),
     Diagnostic(:Ceff, Float"J/K/m^3", OnGrid(Cells)),
     Diagnostic(:k, Float"W/m/K", OnGrid(Edges)),