Save the names of the features for the given feature list to a metadata file.
        Example: `save_feature_names(['num_employees', 'stock_price'], 'company')`.

        Args:
            feature_names: A list containing the names of the features, matching the column order.
            feature_list_id: The name for this feature list.

def save_feature_names(self, feature_names, feature_list_id):
        """
        Save the names of the features for the given feature list to a metadata file.
        Example: `save_feature_names(['num_employees', 'stock_price'], 'company')`.

        Args:
            feature_names: A list containing the names of the features, matching the column order.
            feature_list_id: The name for this feature list.
        """

        save_lines(feature_names, self.features_dir + 'X_train_{}.names'.format(feature_list_id))