KnobGroup where all KnobsIntoHoles have max_kh_distance <= cutoff.

def knob_subgroup(self, cutoff=7.0):
        """ KnobGroup where all KnobsIntoHoles have max_kh_distance <= cutoff. """
        if cutoff > self.cutoff:
            raise ValueError("cutoff supplied ({0}) cannot be greater than self.cutoff ({1})".format(cutoff,
                                                                                                     self.cutoff))
        return KnobGroup(monomers=[x for x in self.get_monomers()
                                   if x.max_kh_distance <= cutoff], ampal_parent=self.ampal_parent)