DatasetCore#match returns Dataset or DatasetCore?

[RubenVerborgh]

The DatasetCore interface description says it returns Dataset, but the description of match says This method always returns a new DatasetCore.

My suggestion would be to make it return a DatasetCore, and to make the equivalent method in Dataset return a Dataset.

[retog]

I think the same applies to the add and delete methods. The advantage of separating DatasetCore and Dataset is that DatasetCore can more easily be implemented and a decorator can be used tomake it a full-fledged Dataset, this advantage is lost if implementors of DatasetCore have to implement Dataset anyway to correctly implemenet the methods.