Add support to allow multiple obo prefixes per ontology

[brianraymor]

Context

See cell-sci-platform.

Previously, there has been one OBO prefix allowed per ontology like HANCESTRO:. In the latest HANCESTRO, there are now imports from the African Population Ontology (AfPO):

    <!-- http://purl.obolibrary.org/obo/AfPO_0000275 -->
    <owl:Class rdf:about="http://purl.obolibrary.org/obo/AfPO_0000275">
        <rdfs:subClassOf rdf:resource="http://purl.obolibrary.org/obo/HANCESTRO_0010"/>
        <obo:AfPO_0000272>18 populations</obo:AfPO_0000272>
        <obo:IAO_0000115>People living in: Algeria OR Egypt OR Libya OR Mauritania OR Morocco OR Tunisia OR Western Sahara</obo:IAO_0000115>
        <rdfs:label>Northern African</rdfs:label>
    </owl:Class>

Both HANCESTRO: and AfPO: prefixes will be allowed by the schema.

As part of multispecies discovery, I'm also seeing this pattern repeat. For example, the Zebrafish Anatomy Ontology (ZFA) imports the Zebrafish Development Stage (ZFS) ontology. A mitigation would be to import the individual ontologies instead of just ZFA.

Add querying support in the OntologyParser API for this use case.