'Delete given metadata key and its value from the catalog.
Parameters
key : str
Metadata key to delete.
Raises
KeyError
Key not found.
VOSError
Key must exist in catalog, therefore cannot be deleted.'

def delete_attribute(self, key):

    if (key in self._compulsory_keys):
        raise VOSError(u'{0}    must    exist    in    catalog,    therefore    cannot    be    deleted.'.format(key))
    del self._tree[key]
