Mypy fix

[JudithBernett]

Fixing all mypy errors. Also restructured class DRP a little bit:

1. model_name is no longer an instance property but a class property. Because it is not possible to have class properties, it was converted into a class method get_model_name()
2. Deleted the SingleDrugModel. It is substituted by a class attribute boolean is_single_drug_model.
3. fixed what's needed as input for all train and predict methods and which format they should have. The DRP class has a required cell_line_input by an optional drug_input because this is None for the single drug models.