Adds a constraint on the reference bus angles.

def _ref_bus_angle_constraint(self, buses, Va, xmin, xmax):
        """ Adds a constraint on the reference bus angles.
        """
        refs = [bus._i for bus in buses if bus.type == REFERENCE]
        Varefs = array([b.v_angle for b in buses if b.type == REFERENCE])

        xmin[Va.i1 - 1 + refs] = Varefs
        xmax[Va.iN - 1 + refs] = Varefs

        return xmin, xmax