Pickle the specified feature list to a file.
        Example: `save_feature_list(project, X_tfidf_train, 'train', 'tfidf')`.

        Args:
            obj: The object to pickle (e.g., a numpy array or a Pandas dataframe)
            project: An instance of pygoose project.
            set_id: The id of the subset (e.g., 'train' or 'test')
            feature_list_id: The name for this feature list.

def save_feature_list(self, obj, set_id, feature_list_id):
        """
        Pickle the specified feature list to a file.
        Example: `save_feature_list(project, X_tfidf_train, 'train', 'tfidf')`.

        Args:
            obj: The object to pickle (e.g., a numpy array or a Pandas dataframe)
            project: An instance of pygoose project.
            set_id: The id of the subset (e.g., 'train' or 'test')
            feature_list_id: The name for this feature list.
        """

        save(obj, self.features_dir + 'X_{}_{}.pickle'.format(set_id, feature_list_id))