Improvements to plant hydraulics model

[kmdeck]

### Tasks
- [ ] Rethink how we handle the root flux in the canopy standalone case. Right now, we specify multiple layers, but use the same soil water potential in each layer. We also need to use the soil conductivity, and not the plant conductivity evaluated at the soil water potential (see #758). Is this data even available for forcing the canopy model? Can we replace this with a single ``root zone" soil conductivity and water potential?
- [ ] Remove prescribed transpiration option
- [ ] Update to use conductance if that is what data we have OR update flux computation to account for differing path lengths
- [ ] Add root tip water potential as prognostic variable?
- [ ] Should we also use harmonic mean for above ground area index?
- [ ] Use the same weibull curve for water stress as for K(psi)?

Other notes from before:

• pass layer widths instead of compartment surfaces
• if there are no stem compartments, we still need to specify stem properties.
• rename midpoiints ->centers and surfaces -> faces to be in line with finite difference conventions
• create a default parameter set