Dataset wrapper API

[dzeber]

For convenience we should wrap test datasets in an object that offers a common API, eg. accessing feature columns and label column, accessing pre-split train/test sets, etc. That way we don't need to pass around information about column names or indexing separately. There should also be methods for reading and writing from files.

We can probably create instances of these objects directly as a part of the dataset setup script #207 and work with these from then on.