Support masked (raster) grids

[benbovy]

This would be useful for modeling non-rectangular domains (e.g., single catchment, volcanic island, etc.) using uniform, raster grids.

This would require an additional node_status label value for skipping nodes that are outside of the domain.

Regarding diffusion_adi_eroder, it only supports solving for the whole grid but I guess one workaround would be to interpolate or extrapolate elevations for each row (column) from the domain boundaries to the grid borders before running the solver in each direction.