Small changes

Changed pickle filename to results-sallenkey.pk, small netlist changes and added plotting of symb results on top of AC results.

Sorry for the map(map(map(...))) code... having a better way to substitute a numpy array in a sympy expression and returning a numpy array would be great. If you have ideas I'm all ears.

Runs fast on my machine (despite the map map map above).

Best,

G

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![sallen-key](https://f.cloud.github.com/assets/5038142/1036531/0ee8022e-0f3f-11e3-9e14-8fa6be0af6aa.png)