Added better explanation of P = FPF' + Q.
Moved conversion of multivariate equations to univariate eqs. to the
math chapter.
Moved the walkthrough of KalmanFilter to an appendix.
My charts were mixing position vs time, which was pretty confusing.
I changed it to position vs velocity, and demonstrated how multipying
the covariances lead to a much better result.
Several changes and clarifications. Added chart showing 2d distribution
of points for multivariate gaussian. Improved some discussions. Fixed
some typos.
