Diagrammatic encoding of equations in Vect

Given the new `@free_diagram` tools in #580, we should be able to do really big systems of equations in Vect. Maybe the Kallman filter is a good example of an application of diagrammatic equations that doesn't need the DEC to explain. We can even compare to the Kallman filter block diagram to see if the diagrams we get match the structure implied by the block diagram.

[0] https://en.wikipedia.org/wiki/Kalman_filter#Details
[1] https://stanford.edu/class/ee363/lectures/kf.pdf