Clean up/split up Exercise 13.18 (robust-statespace) [robperf]

[Richard Murray]

Exercise 13.18 (exercises/robperf/robust-statespace.tex) could be cleaned up and/or split up. The comment in the source files says:

The solution is very long. In the next edition we could consider splitting it up: make one exercise on state feedback in the State Feedback Chapter, make another exercise on the Kalman filter and the output feedback in the Output Feedback Chapter, and make the rest here.

[Richard Murray]

KJA, 4 Jul 2019: I don't think it is a good exercise, much better to compute GoF suggest we skip it.

[Richard Murray]

I moved the exercise into the supplemental exercises section and added a note to indicate that this problem should be split up.