Sort out consequences of deleted opamp example from Ch 9 (xferfcns)

[Richard Murray]

In Ch 9 (xferfcns) there was an example under the "Transfer Functions for Linear Differential Equations" subsection that was about opamp circuits. The example was removed because it didn't really illustrate the ideas listed in that subsection. However, it was referenced in several other places and so now there are broken references.

Use this issue to keep track of the specific problems that this has caused and how we are going to fix them.

[Richard Murray]

Moved to a different location in commit bbead6f. Fixed the issue with removing it (by not removing it!).