Plot frequency response

It may be useful to plot the frequency response of one or more signals.

Both as a sort of frequency analysis and when simulated with an impulse response. In the former case, a window should be applied.

added Enhancement Simulation results GUI labels

assigned to @oscgu95

assigned to @petka86

Isn't this exactly what freqz do? (except it doesn't take many plots. You have to call it once per plot, with the same ax argument).

Yes, but the idea is to be able to do it in the Sim-GUI. Say you simulate from the SFG GUI with an impulse (or a random signal) and then you want to see the frequency response of the nodes.

Some problems:

1. You do not really want a freqz-version of every signal all the time. (And it may be time-consuming to compute for large inputs.)
2. You may want to look at both phase and magnitude with different scales (this is actually a problem already, maybe that can be solved though)
3. For some inputs you would like to window the results before plotting.

I'm now thinking that maybe a tabbed design, with one time-domain tab and one frequency-domain tab may be good (with different canvases/axes in each). And then only generate the frequency-response on a per-signal basis when it is activated the first time. Will still be a problem for windowed data though, but that can be solved later.

mentioned in issue #209