API: add sub-packages to sklearndf.wrapper

[j-ittner]

This PR makes package sklearndf.wrapper easier to navigate, and increases de-coupling of the stackable learners implementation.

• moves some classes in module sklearndf.wrapper to sub-packages sklearndf.wrapper.stacking and sklearndf.wrapper.numpy
• this improves package navigability and achieves better de-coupling of the underlying code, esp. for stacking
• moves classes StackingClassifierWrapperDF and StackingRegressorWrapperDF to package sklearndf.wrapper.stacking