Validate data (rows length, categories=entities, square).

def validate(self):
        """Validate data (rows length, categories=entities, square)."""
        validate_rows_length(self.data, self.columns, exception=self.error)
        validate_categories_equal_entities(self.categories, self.entities,
                                           exception=self.error)
        if self.square:
            validate_square(self.data, exception=self.error)