For default columns in components: Make sure they are consistent with the spec in the default metadata
For other columns: Make sure they are consistent with the spec in the ontology
card = col.common_props.get('dc:extent')
assert (card == 'multivalued' and col.separator) or \
(card == 'singlevalued' and not col.separator) or \
(card is None), 'z'
See https://github.com/cldf/cldf/issues/106