Add water balance (bucket scheme + Richard's equation)

Also includes some other refactoring and clean up.

The only breaking change is that delta/divergence variables are now called ∂u∂t instead of du where u is the prognostic variable. This looks better with other prognostic variable names, like sat in WaterBalance :)

Units are also now mandatory on Forcing types, and they are used to explicitly require forcings with a particular unit where suitable.