Set sqlite cursor on first call, not on instantiation

[kkaris]

This PR fixes a subtle issue that arises from how the bio_ontology is imported: because an instance of the BioOntology or the SqliteOntology is created when importing bio_ontology from the module, the thread that does the import is the one that owns the self._local attribute. This is apparently a different thread than the ones makings calls to the sqlite database later in at least one multi threading setup that uses the SqliteOntology.

The fix sets self.cur as a property instead of an attribute. On first access to the cursor by a thread, that thread sets its local cursor.