Document variable naming

[VEZY]

• A variable should be named using the standard name used in the community.
• It shouldn’t include the variable unit, as it may depend on the unit of the model inputs.
• The unit and description of the variable should be documented in the model structure documentation. If the unit depends on the inputs of the model, state it.
• Variables should be snake case, e.g. leaf_area, unless widely used in the community, e.g. LAI
• Variable names shouldn’t be abbreviated, unless widely used by the community, e.g. leaf area should be named leaf_area, not la. LAI should be fine, but leaf_area_index is better.
• A variable should be named with a generic name, e.g. temperature instead of temperature_leaf, so the same name can be used at different scales. One exception is when a variable integrates the values from other scales; e.g. Rm_plant is fine if it is computed using the sum of all Rm of the organs in the plant.

[Samuel-amap]

Closing this issue, it is redundant wiht #132