Implement the option to target a specific ontology when resolving terms

With the current implementation of the OntologyResolver for nexus-store (https://github.com/BlueBrain/nexus-forge/blob/master/kgforge/specializations/resolvers/ontology_resolver.py), one can resolve terms from a given store (i.e. by specifying the bucket).

Since multiple ontologies may be contained within a single bucket, it would be great to be able to target a specific ontology (e.g. by its identifier or label) within a given bucket. This could be implemented by leveraging the **isDefinedBy** property present on the classes of the ontology.