Add momentum to water dynamics

[alice-i-cecile]

The idea here is:

1. This may reduce strange periodic oscillations (Lateral diffusion of water can cause distracting and glitchy oscillations #845), enabling us to increase water flow rate.
2. Straight rivers should flow faster.
3. Overflow side-branches of rivers should not absorb much of the water flow.

Precipitation modelling paper: https://www.usgs.gov/software/precipitation-runoff-modeling-system-prms
Soil water modelling paper: https://pubs.usgs.gov/tm/2005/tm6A16/PDF/TM6-A16ch2.pdf
More in depth overview of soil water model: https://pubs.usgs.gov/tm/tm6d1/pdf/tm6d1.pdf