Add ontology checks

[ielis]

Perform compatibility checks when loading the MinimalOntology.

The checks include:

• ensure we have vertices for all subjects and objects of the edges,
• check that the ontology graph is connected
• check that the ontology graph is simple - no self loops and no duplicated edges. Note that edges of different types are allowed between the same nodes.

The loader allows opting out of the duplicated edges feature by setting discardDuplicatedRelationships(true) in OntologyLoaderOptions.