PlantHydraulics Model Simplification

[Espeer5]

Is your feature request related to a problem? Please describe.
To make progress toward #596, we need a simplified PlantHydraulics model designed specifically for a Big leaf model, which doesn't need to carry around vectors of layer heights.

Describe the solution you'd like
Build a BigLeafHydraulics model which is the reduction of the current PlantHydraulics model to a single leaf and stem layer.

Describe alternatives you've considered
One possible alternative was to reduce the plant hydraulics model to a multilayered model where each layer was the same height, but this was spurned in favor of the explicit big leaf model since our current intended use case is a big leaf model only.

Additional context
This hydraulics model will be coupled to a new Biomass model which will move all of the area indices out of the hydraulics model. The choice to remove n_stem, n_leaf, and the other parameters which support a multilayered PlantHydraulics model will make the interface between these two modules much cleaner.