Add __iter__ method to Dataset

[kvjrhall]

Motivation

It is convenient to be able to iterate over all triples in a Graph instance without invoking graph.triples((None, None, None)). When working with a small instance of Dataset, there are occasions where one will want to observe all of the quads in a similar manner.

Notes

This is trivial to implement by simply having an introduced __iter__ method invoke self.quads((None, None, None, None)) in exactly the same way that it is done in Graph.

[nicholascar]

This is trivial to implement…

Then please do submit a Pull Request for this! We can then review it the implementation causes an problems.