Connect plants to soil moisture

**Is your feature request related to a problem? Please describe.**
Plant water uptake and soil moisture modelling are currently only directly linked; soil moisture is reduced based on the evapotranspiration of the preceding month and plants assume unlimited water availability. This might lead to an overestimate of NPP.

**Describe the solution you'd like**
Ideally we would like to run plants, hydrology, and abiotic model at a high temporal resolution to avoid this mismatch. This will likely not be computationally feasible. I would at least like to implement a function that makes plants see how much water there is available, for example by considering potential and actual evapotranspiration.

Start by reviewing how the plants, hydrology, and abiotic models currently exchange soil moisture and evapotranspiration information. Define how plant water availability should be represented using potential and actual evapotranspiration, then verify that plant water uptake and NPP no longer assume unlimited water.

Written by the indexing model from the issue text.