Hydrology - set boundaries for river flow

At the moment, the river flow start only from rainfall and accumulates in the lowest grid cell. We need to make sure that water leaves the grid and that inflow from outside the grid is accounted for.

For the outflow, a fixed rate could be removed from the lowest grid cell per timestep.
Inflow is a bit more tricky as we need some information about the catchments and where water could flow in our model grid. A pre-simulation analysis or a larger area could help to find a rate of inflow and locations where this happens.