Additions to MKF chapter.

Several changes and clarifications. Added chart showing 2d distribution
of points for multivariate gaussian. Improved some discussions. Fixed
some typos.
Renamed chapter.

Renamed to One Dimensional Kalman Filters
Renamed variables to improve understanding.

I used p and pos for the belief in position, which can be a bit
confusing.
update() and predict() alter in place.

I made the update() and predict() functions alter the belief array
in place rather than returning the value in a new array. I find this
far more readable and less error prone.