Missing code documentation in `sentinel_2` class

The preprocess_date method of sentinel_2 class in Sensors/Sentinel_2.py lacks a comprehensive docstring (input parameters, return value, conditional branches).

https://framagit.org/iota2-project/iota2/-/blob/develop/iota2/Sensors/Sentinel_2.py#L252-L361

It could be made easier to read and maintain by decoupling the directories management (270, 280, 286, 306, 332, 340, 358) and the business logic.