Consider alternatives to extrapolations in hydro P-V curves

[rgknox]

Plant hydraulics relies on pressure-volume functions in order to transform between the two. We have a few different functions to choose from, TFS (Christofferson), Van-Genuchten and Campbell-Clapp-Hornberger. However, the user can also specify that we extrapolate the existing functions beyond a certain range (both high and low) of relative water content, in the interest of avoiding numerical irregulaties.

PR #737 introduced a potential alternative approach to extrapolation, but it was withheld from the PR because it was out of scope. There, @JunyanDing explored that we cap pressures and water contents at saturation and not let it go beyond, and to avoid extapolation at the lower end of the range (towards residual), thereby using the original functions.

See changes here: JunyanDing@b53d685

This issue was written to document that this needs more investigation and follow up.