Check saddle solution electric generator exercise in Ch 5 [dynamics]

[Richard Murray]

At some point we got an errata report for Exercise 5.6 (exercises\dynamics\powergrid.tex) saying that "we need to shift $\varphi_0$ in part (c)" (which defines a curve for the saddle). We can't find the original errata submission, but we should make sure that the equation is correct. The solutions show that the equations of motion are satisfied by given curve, but don't verify that the curve actually goes through the saddle point.

At a minimum, we should update the solutions to verify that the curve goes through the saddle point.

[[e-mail]]

Corrected various mistakes and made minor changes

[Richard Murray]

Assigning to Richard to check solution.

[Richard Murray]

Fixed by Karl and confirmed by RMM on 10 June 2019.