Evaluates F(x).

def _evaluate_function(self, Ybus, V, Sbus, pv, pq):
        """ Evaluates F(x).
        """
        mis = multiply(V, conj(Ybus * V)) - Sbus

        F = r_[mis[pv].real, mis[pq].real, mis[pq].imag]

        return F