Load observing mode validator

def load_mode_validator(obs_mode, node):
    """Load observing mode validator"""

    nval = node.get('validator')

    if nval is None:
        pass
    elif isinstance(nval, str):
        # load function
        obs_mode.validator = import_object(nval)
    else:
        raise TypeError('validator must be None or a string')

    return obs_mode