add option to import images into a folder, not at the root of the corpus

    import_target = iiif_parser.add_mutually_exclusive_group(required=True)
    import_target.add_argument(
        "--corpus-id",
        help="UUID of the corpus to import images to.",
        type=UUID,
    )
    import_target.add_argument(
        "--parent-folder", help="UUID of the folder to import images to.", type=UUID
    )

• Checks that the given UUID points to a folder type element
• Checks that the element exists