Surfacewater: Inconsistent methods to calculate covered area

[philippkraft]

At a number of places, cmf needs to now which fration of a cell is covered by surfacewater, eg. Shuttleworth-Wallace to route ground evaporation either from the surface water or the first layer.

The cmf::upslope::Cell::surface_water_coverage function uses the undocumented "cmf::upslope::Cell::surface_amplitude" value. The puddle depth of the surfacewater has a similar function and is much cleaner.