Improved percolation and minimum SWC

[dschlaep]

Incorporating updated SOILWAT2 (see https://github.com/DrylandEcology/SOILWAT2/pull/293)

--> this version leads to different simulation output

--> Improved unsaturated percolation and behavior of minimum SWC

• Improved limit for swc_min
• Unsaturated percolation rate is now adjusted for swc_min
• Bare-soil evaporation, transpiration, and hydraulic redistribution no longer extract soil moisture held below swc_min