Add support for negative inflow to kinematic_wave

[kordejong]

Lateral inflow represents the addition of water (e.g. precepitation), or removal of water (e.g. infiltration, pumping). Very comparable with accu_threshold. The outflow passed in (negative inflow) is a maximum / potential value and may be different from what is actually removed from the cell.

We could add a second return value containing the actual amount of water removed, given the actual amount of water available in each cell. Until then, it is impossible for users to check the water budget when less water is removed than potentially can be removed. See #527

Lateral inflow can be negative. Make sure the actual outflow (negative inflow) is not larger than the actual amount of water in the cell.