Added `Obj.get_data()` method to return only, data-associated keys.

Running `get_data()` on any `graph_obj` does the following:

1. Convert all entries/key-values to a `graph_obj`.
2. If you can, run `get_data` on each entry/key-value.
3. If the key-value is associated with data, keep it!
4. Remove empty lists and dicts.
5. Shell a dictionary layer if it only includes one entry.