[feature] Grassim - add documentation about variables_to_save()
Summary
Add documentation for using variables_to_save()
Current behavior
The possible values of variables_to_save() are not explicitly listed anywhere. The usage of this file is not mentioned in documentation
Desired behavior
Add documentation on how to use variables_to_save.csv. The right place for this documentation for now is in https://gitlab.uliege.be/deal-public/pase/-/tree/main/MODULES/CROPS/GRASSIM. Maybe all possible values should not be explicitly listed but at least mention that they should be arguments of corresponding classes.