Compute the N trajectories that constitute the current genealogy.

        Compute and add attribute ``B`` to ``self`` where ``B`` is an array
        such that ``B[t,n]`` is the index of ancestor at time t of particle X_T^n,
        where T is the current length of history.

def compute_trajectories(self):
        """Compute the N trajectories that constitute the current genealogy.

        Compute and add attribute ``B`` to ``self`` where ``B`` is an array
        such that ``B[t,n]`` is the index of ancestor at time t of particle X_T^n,
        where T is the current length of history.
        """
        self.B = np.empty((self.T, self.N), 'int')
        self.B[-1, :] = self.A[-1]
        for t in reversed(range(self.T - 1)):
            self.B[t, :] = self.A[t + 1][self.B[t + 1]]