Add exercise on feedforward control for web servers [loopsyn]

[Richard Murray]

Karl suggested that we might add an exercise on the use of feedforward in web servers (cloud computing farms) based on some recent results of a student.

This is optional and relatively low priority.

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Have added Exercise 12.5

[Richard Murray]

Karl, 4 Jul 2019: I think exercise 12.5 (loopsyn) is there already, may be an old issue.