I don't like doing this inline import, but I'm not sure how to get around the circular reference otherwise (`objects.Network` requires `EmptyEnsemble`, `EmptyEnsemble` requires `objects.Ensemble`, so you can't just import EmptyEnsemble at the `objects` level).  If anyone knows some clever python trickery to get around that, that'd be great.  We could split Network/container stuff into a different file from nengo objects, but I didn't want to make a big change like that unless everyone was on board.