bug in io.Reader().run()

[artgoldberg]

io.Reader().run() will fail if given group_objects_by_model=False, because the use of objects after reading the model with

objects = super(Reader, self).run()

depends on objects being grouped by model.

[jonrkarr]

Fixed.

If group_objects_by_model is False, the objects are now flattened to a list.