Support for pandas _query_iterator

[TomAugspurger]

It would be nice if this worked:

gen = pd.read_sql_query(complex_sql, engine, chunksize=10)
odo.into('data.hdf5::/test', gen)

Right now I don't think odo is aware that gen, a _query_iterator is just an iterator of DataFrames. You get back that _query_iterator from pd.read_sql_table or pd.read_sql_query when you specify a chunksize.

I used a workaround Matthew suggested on the mailing list

def f():
    gen = pd.read_sql_query(complex_sql, engine, chunksize=10)
    for df in gen:
        yield df

odo.into('data.hdf5::/test', odo.chunks(pd.DataFrame)(f))

For testing, this doesn't even need to be a complex query, you can just use pd.read_sql_table with a chunksize.