All ontology OBO files should be downloaded and placed in this directory.
Ontologies currently required:
    - Disease Ontology (http://disease-ontology.org/)
    - Cell Ontology (https://bioportal.bioontology.org/ontologies/CL)
    - Uberon (http://uberon.github.io/)
    - ChEBI (https://www.ebi.ac.uk/chebi/)
    - Experimental Factor Ontology (http://www.ebi.ac.uk/efo/)
    - Unit Ontology (https://bioportal.bioontology.org/ontologies/UO)
    - Cellosaurus (http://web.expasy.org/cellosaurus/)
