ders that do not represent
    classes, they can be ignored by explicitly listing the subfolders of
    interest using the classes argument.
    Args:
        datadir (str): path to folder
        classes (list): list of classes (subfolders to consider).
                        This is simply supplied as the categories argument
                        to sklearn&#39;s load_files function.
        max_features (int):  maximum number of unigrams to consider
                             Note: This is only used