Loading ontologies that do not import skos fails

[francescalb]

Non-EMMO-based ontologies might not import skos.
In ontology.py default annotations are set as:
'''

Default annotations to look up

DEFAULT_LABEL_ANNOTATIONS = [
'http://www.w3.org/2004/02/skos/core#prefLabel',
'http://www.w3.org/2000/01/rdf-schema#label',
'http://www.w3.org/2004/02/skos/core#altLabel',
]
'''

i.e. import of both skos and rdf-schema is expcted. This needs to be solved.