Reinsert figure + add alpha to figure + update solutions in op amp oscillator exercise [dynamics]

[Richard Murray]

Exercise 5.13 (exercises/dynamics/opamp-nlosc.tex) is missing a figure that used to be there. When we reinsert the figure, we should also consider adding the parameter $\alpha$ that is referred to in the exercise.

In addition, there is a note saying that the solutions to this exercise need updating.

[Richard Murray]

Figure re-inserted in commit bafcca6

[Richard Murray]

Figure updated in commit 564275a.