DrylandEcology/rSOILWAT2: v5.4.0

SOILWAT2 is updated to v6.7.0 which fixed vegetation establishment. This version produces identical simulation output as the previous release under default values (i.e., vegetation establishment is turned off). Functionality to calculate and output establishment/recruitment of species now works and is covered by tests (issue #225, @dschlaep). Note that this functionality assesses yearly the chances of species to recruit/establish based on simulated daily conditions; however, establishment/recruitment outcomes are not utilized to inform the simulation. Full Changelog: https://github.com/DrylandEcology/rSOILWAT2/compare/v5.3.3...v5.4.