Returns non-linear constraints for OPF.

def _nln_constraints(self, nb, nl):
        """ Returns non-linear constraints for OPF.
        """
        Pmis = NonLinearConstraint("Pmis", nb)
        Qmis = NonLinearConstraint("Qmis", nb)
        Sf = NonLinearConstraint("Sf", nl)
        St = NonLinearConstraint("St", nl)

        return Pmis, Qmis, Sf, St