Constrains the difference b/w primary and secondary to be < diff

def constrain_oddeven(self, diff, **kwargs):
        """Constrains the difference b/w primary and secondary to be < diff
        """
        if 'odd-even' not in self.constraints:
            self.constraints.append('odd-even')
        for pop in self.poplist:
            if isinstance(pop, EclipsePopulation_Px2):
                pop.constrain_oddeven(diff, **kwargs)
        self.oddeven_diff = diff