fix: examples in kalman_filter.py

[Erotemic]

The code in the Kalman filter example was broken in multiple ways. There were syntax errors, array shape errors, and undefined name errors.

I think I've fixed them in a reasonable way. I've also added a seeded random number generator to make the example deterministic. The only thing I'm not sure about is when I define dt = 1 as a constant, and if cv is the best name for the KalmanFilter instance in this example. Perhaps f is better. Please advise.

I'll also note that perhaps some of these examples might be better written as a doctest. The standard library doctest runner can be a bit annoying, but my xdoctest library works very well and would help ensure that all of the examples actually run.

I can convert this particular example if desired. Let me know.