Pass nadir flag in isd_generate (#624)

    if len(args.input) == 1:

        try:

            file_to_isd(args.input[0], args.out, radii, kernels=k, log_level=log_level, compress=args.compress, only_isis_spice=args.only_isis_spice, only_naif_spice=args.only_naif_spice, local=args.local)

            file_to_isd(args.input[0], args.out, radii, kernels=k, log_level=log_level, compress=args.compress, only_isis_spice=args.only_isis_spice, only_naif_spice=args.only_naif_spice, local=args.local, nadir=args.nadir)

        except Exception as err:

            # Seriously, this just throws a generic Exception?

            sys.exit(f"File {args.input[0]}: {err}")