Replace WHC-approach to water balance modelling

[stineb]

Resolve WHC with its components:

• root zone water content at wilting point
• root zone water content at field capacity
• root zone capacity

... from which an effective root zone depth is calculated. Best is to read in soil texture (fraction silt, clay, sand, gravel) as a site-level parameter and calculate the critical water content quantities using pedotransfer functions. The root zone capacity will be defined by Beni's global maps.

@stineb will have to do this. To be adopted by @fgiardin and Jiarui.

This will trigger an update of the rsofun version and benchmarking will have to be run.