def _get_files(extension, path):
        """
        Returns a sorted list of all of the files having the same extension under the same directory
        :param extension: the extension of the data files such as 'gdm'
        :param path: path to the folder containing the files
        :return: sorted list of files
        """
        # retrieves the files sharing the same extension
        files = []
        for file in os.listdir(path):
            if file.endswith(extension):
                files.append(os.path.join(path, file))
        return sorted(files)
Returns a sorted list of all of the files having the same extension under the same directory
        :param extension: the extension of the data files such as 'gdm'
        :param path: path to the folder containing the files
        :return: sorted list of files