backwater water surface computation?

[amoodie]

In _accumulate_free_surface_walks(), where the dH values are set within the delta I think it could be good to either add a comment, or open an issue to indicate that the if-else could be further split up to incorporate water surface assignment based on a backwater profile. This is something I've never played with, but was present in the Matlab implementation, @ericbarefoot have you?

The original Matlab implementation included commented out code that would enable a backwater computation of the water surface.

None of us have experimented with this, but it could be something to implement via a separate function or if-then statement and a flag in the configuration file.