Delete one or more relationship, by index, from the extent

        index - either a single index or a list of indices

def remove(self, index):
        '''
        Delete one or more relationship, by index, from the extent

        index - either a single index or a list of indices
        '''
        raise NotImplementedError
        if hasattr(index, '__iter__'):
            ind = set(index)
        else:
            ind = [index]

        # Rebuild relationships, excluding the provided indices
        self._relationships = [r for i, r in enumerate(self._relationships) if i not in ind]