Changed equations to use Gu instead of Bu.

I switched my code over to this nomenclature a while ago, but have not
updated the book for the switch.

As an aside, the code for the Kalman filter class in theb book is now
quite dated. I need to incorporate filterpy library more fully.
Symbology changes.

Working on finalizing the symbology used in this book. Added appendix
to document what other books in the field use for the Kalman filter
equations.