Compatibility of ep.pp.knn_impute with different datatypes

[eroell]

Description of feature

Follow the template workflow of #835:

• introduce singledispatch to check for allowed data types, or raise an error if others are encountered
• adapt the tests to check these data types

[eroell]

At a first glance, daskml does not have a K-neighbors imputer like sklearn.

Having this here would be a significant addition. This is a non-trivial effort, and some engineering is required for this scaling improvement.