Aquifer

Aquifer water-storage update for the case when the water table lies below the soil column. The rate of change of aquifer water dW_a (kg/(m^2*s)) is the balance of recharge into the aquifer and sub-surface drainage out of it (Oleson et al. 2010, CLM Tech Note Ch. 5, Eq. 5.143). Migrated from UrbanCanopy.jl@9200edd4 src/hydrology.jl:791 (`AquiferWaterBalance` @component). Observed-only: both inputs are parameters and dW_a is given by a single algebraic equation.