be lazy in downloading datasets and splits

[amueller]

I think the get_dataset and get_task functions should not download any big items. That allows you to inspect the attributes of the data without having to download possibly large amounts of data.

We can just download the data in the get_data method - similarly, in get_task we shouldn't download the splits, but only download them later when they are needed.

[mfeurer]

Functionality for lazy downloading of datasets added in #644, get_task still needs a download flag.

[mfeurer]

Functionality for lazy downloading of tasks added in #659.