Add test for delete

[bgajdero]

The SPARQLDict._delete() and NodeGraph.delete() have refs parameter.

If refs=True, all triples linked to the node being deleted. For a node with inst_id=id_value all [id_value pref obj] and [sub pred id_value] triples are deleted.

If refs=False, linked nodes are not deleted, only the main node is deleted. For a node with inst_id=id_value all [id_value pref obj] triples are deleted.

Add tests 45, 46, and 47 to test_sparql_dict.py

Add similar tests 60, 61, 62 to test_unit_test_node1.py