Make dispatched builders instantiate graphs instead of modifying params
Address TODO for missing disconnected nodes

+    # I think we don't need to consider disconnected nodes, since that
+    # is impossible to represent in straight RDF. That is, RDF has a
+    # concept of disconnected triples, but not terms, which are
+    # properly analogous to nodes in networkx
Refactor function names for consistency with other modules
Add missing path param to a writer
Implement to_rgmlgraph()