Add save method to Dataset class

I would just add them to loaders (despite not being really a loader) in the same way a loader is currently implemented (so with can_save and save methods). Not all loaders should support saving (they could always return False from can_save, and raise NotImplementedError from save).

This could allow one to save D3M datasets:

• destination URI should still point to a datasetDoc.json file
• nor the file nor its parent directory should exist
• we should map our metadata to D3M dataset qualities (#227 (closed))

An interesting question is how to store our Dataset class into a CSV file. One option could be that only a dataset with one 1D or 2D tabular resource could be supported. And caller should make sure a dataset is in this format.

(Other options could be that caller could specify which resource to save to a CSV file, but this would complicate the API as we would have to add an extra argument to save method.)