added error when output directory is not empty

    # parse configuration

    cfg = config.TestConfig(args.config)

    # check if output folder is empty

    if cfg.output_path.is_dir():

        file_list = [

            f.resolve().absolute()

            for f in cfg.output_path.iterdir()

        ]

        if len(file_list) != 0:

            raise ValueError("Output folder is not empty. Please delete or move files and folders.")

    # set up processing chains

    chains.init_processing_chains(cfg)