Performs a Q iteration, updates Vm.

def _q_iteration(self, Q, Bpp_solver, Vm, Va, pq):
        """ Performs a Q iteration, updates Vm.
        """
        dVm = -Bpp_solver.solve(Q)

        # Update voltage.
        Vm[pq] = Vm[pq] + dVm
        V = Vm * exp(1j * Va)

        return V, Vm, Va