Need Explanations and Comments in smoothed_cch water transfer functions

[rgknox]

The "smoothed_cch" water transfer functions (water retention curves and conductances) could use some comments that define the variable names and generally describe what is going on and where the functions came from.

For instance here:
https://github.com/NGEET/fates/blob/sci.1.74.0_api.35.0.0/biogeophys/FatesHydroWTFMod.F90#L1013-L1019