[Task]: Make `Assimilation` differentiable and efficient

The module [Assimilation ](https://github.com/ajwdewit/pcse/blob/master/pcse/crop/assimilation.py) has several classes. Only `WOFOST72_Assimilation` should be changed which is related to wofost72. See explanations and requirements in https://github.com/WUR-AI/diffWOFOST/issues/39.