[fix] GrasSim: clip soil water content min value to zero

Summary

Gras-Sim soil water content is clipped between the wilting point and water saturation. Clipping to the wilting point overestimates water content during droughts and is not supported by Kokah et al. (2023) or Ruelle et al. (2018). The minimum should instead be set to zero.

Sources

Kokah 2023 : https://doi.org/10.1016/j.eja.2018.06.010 Ruelle 2018 : https://doi.org/10.1016/j.jafr.2023.100875