Plant hydraulics updates

[ckoven]

Following up on today's FATES call, several developments are either proposed or underway for plant hydraulics in FATES. Just want to list some of them here, for reference and tracking of their evolving status. Feel free to add to and/or update this thread as things proceed. cc: @JunyanDing @xuchongang @pnlfang

• reworking some of the root-layering so as to generalize away from the requirement that every soil layer has a corresponding root layer.
• updates to P-V and PLC curves.
• removing the assumption that leaf internal relative humidity equals 100% when leaf water potentials become dry.
• incorporation of FETCH2 dynamics to more finely resolve canopy water potential gradients.
• possibly adding lower-complexity hydraulic models, e.g. similar to CLM5 PHS, to FATES.
• @JunyanDing didn't get to this today but is also adding size-resolved cohort-specific rooting depths.