NL5: borrowing from "digital" world

A new feature is available in NL5 ver. 1.91: Z-transform method for low signal AC analysis.

It is an interesting one: z-domain analysis is typically used for discrete time signals, which is typical for digital circuits, but definitely not for continuous "analog" world. However, it happens that it might be working good enough for continuous periodical (or quasi-periodical) signals as well. Of course, there are some limitations, and the user should be very careful looking at the results.

Simulation time saving is huge: z-transform would be hundred or even thousand times faster than standard "Sweep AC source" method. Such an improvement is probably worth spending some time to learn basics of discrete time analysis, or at least what Nyquist frequency is.