Add support for simple persistent storage in the rdflib backend

[jesper-friis]

This will solve a need in OTEAPI that for testing would need a server-less triplestore backend that is persistent within a session.

The idea is to utilise the base_iri argument in the rdflib backend. If provided, load the specified storage at initialisation.
Add a close() method that writes all current triples to the storage.

The reason for using the rdflib backend is that it is the most well-developed. It is also quite efficient, except for SPARQL...