Computes the IMM's mixed state estimate from each filter using
        the the mode probability self.mu to weight the estimates.

def _compute_state_estimate(self):
        """
        Computes the IMM's mixed state estimate from each filter using
        the the mode probability self.mu to weight the estimates.
        """
        self.x.fill(0)
        for f, mu in zip(self.filters, self.mu):
            self.x += f.x * mu

        self.P.fill(0)
        for f, mu in zip(self.filters, self.mu):
            y = f.x - self.x
            self.P += mu * (outer(y, y) + f.P)