standard interface to KF, UKF, and EKF

[rlabbe]

The EKF does not implement batch_filter or rts. There are probably similar instances of differences between the classes.

[robots]

One more difference is update()... Ukf takes **hx_args, while Ekf takes hx_args=() Another difference is UKF's x vector - zeros(dim_x), ekf has this vector transposed.