Zipped benchmark datasets

[csala]

The make_datasets function should accept a new argument zipped (default to False) which, if True creates the dataset as a .zip file instead of a plain folder.

The Dataset class should also be modified to be able to load datasets that are stored as a zip file without any additional input: if the dataset argument is a path to a file and its name ends in .zip, decompress it in memory before loading it.