Given keyword args, return optional_attributes to be loaded into the GODag.

def _get_optional_attrs(kws):
        """Given keyword args, return optional_attributes to be loaded into the GODag."""
        vals = OboOptionalAttrs.attributes.intersection(kws.keys())
        if 'sections' in kws:
            vals.add('relationship')
        if 'norel' in kws:
            vals.discard('relationship')
        return vals