json export should be the same as for frontend (with direct parents)

The frontend at chebifier.hastingslab.org offers a download in JSON format. In the Python package, we also offer to save results to a JSON file, but it does not follow the same structure. To make things less confusing, we should offer the same functionality for both. In particular, the frontend separates "all predicted superclasses" from "direct predicted superclasses" which is useful to have.