vp-checktheory needs to change parsing of fit

Created by: wilsonmr

at the moment supplying a fit to vp-checktheory doesn't work because we have:

group.add_argument(
        '--fit',
        type=str,
        nargs=1,
        help=(
            "Name of a fit from which to parse `theoryid` from, instead of "
            "supplying theoryid on command line"
        ),
        default=None
    )

the nargs=1 means that args.fit is a list of length 1 rather than just the string, getting rid of this fixes it.. PR to follow