Adds a file handler to the root logging.

    :param log_fn: the name of the log file.

    :type log_fn: str

def add_file_handler_to_root(log_fn):
    """Adds a file handler to the root logging.

    :param log_fn: the name of the log file.

    :type log_fn: str

    """
    file_handler = logging.FileHandler(log_fn, mode="w")
    file_handler.setFormatter(logging.Formatter(
        fmt="[%(asctime)s %(name)s %(levelname)s] %(message)s",
        datefmt="%Y-%m-%d %H:%M:%S",
    ))
    logging.root.addHandler(file_handler)