Base validation + entities = rows + columns.

def validate(self):
        """Base validation + entities = rows + columns."""
        super().validate()
        nb_entities = len(self.entities)
        if nb_entities != self.rows + self.columns:
            raise self.error(
                'Number of entities: %s != number of rows + '
                'number of columns: %s+%s=%s' % (
                    nb_entities, self.rows, self.columns,
                    self.rows + self.columns))