DrylandEcology/rSOILWAT2: v5.1.0

SOILWAT2 is updated to v6.3.0 with improved percolation and behavior at minimum soil water content (https://github.com/DrylandEcology/SOILWAT2/releases/tag/v6.3.0) including: Unsaturated percolation rate is now adjusted for swc_min, i.e., percolation rate is smaller at very low moisture levels. Bare-soil evaporation, transpiration, and hydraulic redistribution no longer remove soil moisture held below swc_min. Lower limit of swc_min is now set at -30 MPa. This version produces slightly different simulation output from previous versions v5.0.