Improve processing of lists in RDF queries

[argenos]

          If you are not recursively parsing lists of lists, you can pass the list's head (should be a [`BNode`](https://rdflib.readthedocs.io/en/7.1.1/apidocs/rdflib.html#rdflib.BNode)) into [`rdflib.graph.Graph.items()`](https://rdflib.readthedocs.io/en/7.1.1/apidocs/rdflib.html#rdflib.graph.Graph.items) to get the list back. So the `get_list_from_ptr` is rather redundant, but this maybe a separate issue.

If you want to parse list of lists recursively, then there's rdf_utils.collection.load_list_re in rdf_utils.

Originally posted by @minhnh in #12 (comment)