[feature] Gras-Sim model docstrings

Summary

Add Grassim documentation using doctrings

Current behavior

No documentation of the Gras-Sim model

Desired behavior

Add doc in Gras-Sim codes (Plant.py and Soil.py) :

• Method description, with units
• Parameter description and type

Sources

https://peps.python.org/pep-0257/

Linked features or branches

https://gitlab.uliege.be/deal-public/pase/-/wikis/documentation/dev-guide