Add a method that returns a generator

[cmungall]

Currently when executing goals that return many results, the Query object will keep accumulating results until all results have been found. There is no way to stream the results.

Ideally we could write:

for r in q.results():
     # do something with r

See https://wiki.python.org/moin/Generators

[cmungall]

oh hold on, I see Query implements __next__, so the query object can act as an iterator, I see

[cmungall]

to properly implement an iterator