hydraulics sensitivity to crown depth

[rgknox]

FATES currently uses two different ways to estimate crown depth, for different routines.

For calculating crown fires and snow occlusion of leaves, it uses the fates_fire_crown_depth_frac parameter, where:

crown_depth = crown_depth_frac * height

For hydraulics, it assumes the crown depth is the minimum of 1 meter and the height of the plant:

crown_depth = min(1.0, height)

Through efforts on #738, @JunyanDing and I tested the effects of unifying the scheme, so that hydraulics use the first approach. This change will be withheld from PR #738 , but we wanted to discuss these alternatives and if/how we should change. The differences in results between using the two schemes were noticeable. The following is a simple comparison at BCI, between master, and a version that migrates hydrualics to use the first formulation of crown depth. ST3 mode is also turned on.

rootdepth-v3-st3_plots.pdf

[xuchongang]

@rgknox , I think it would be great to make hydro code consistent with the other codes. It should not have much impact on the hydrodynamics.