getState returns the state of [pos, vel, accel] at time t
 
    Arguments:
        time - scalar or vector of times
 
    Note that the trajectory state is only defined for 
    times between zero and the total duration.
 
    Example
       % Plot position over entire trajectory
       t = 0:0.01:trajectory.getDuration();
       [pos, vel, accel] = trajectory.getState(t);
       plot(t, pos);