Enhance `still_water_surface` and `bathymetry`

[ranocha]

We could consider adding

varnames(::typeof(still_water_surface), equations) = ("η_0",)

and similarly for bathymetry further down. If we do this, we should also add them to the conversion_functions in the unit tests.

Originally posted by @JoshuaLampert in #127 (comment)