adding fft skip (zoom out)

[jacobagilbert]

This is an updated version of #96 I have been using for a bit. There was some discussion back then of the 'right' way to handle zooming out, and some concern that simply striding over the input data was not ideal but I'm submitting this again in case its useful because:

1. You just can't do this right now at all
2. Its super fast still because its not computing an FFT on every data point; one per visible column
3. There is not currently a configuration panel or settings menu to select different decimation behavior
4. I fixed the stuff that did not work correctly in the other PR (time-scale, sigmf annotations)

[devnulling]

just tested this out, seems to work fairly well!

[Teque5]

let's get it merged!