Enable subframe sampling rates for FFT Widget

[jackiryan]

Legacy LiveView computed the FFT at 4 different sampling rates. The image can be sampled at a rate "faster than the frame rate" by selecting areas of pixels and treating them like a time sequence. This is because the data in the camera link (or similar serial) channel is sent at a constant clock frequency. That is, for a 10 MHz clock rate camera link signal, each pixel represents a sample spaced 0.1 us apart.

In this way, selecting one column of the image samples at a rate of the clock rate divided by the number of columns. For example, in the previous example with a clock rate of 10 MHz, using a frame geometry with a width of 640, we get a sampling rate of 15.6 kHz when treating a single column of the image as a frame. Note that this does not work for selecting rows since the serial channel sends items in the row sequentially, at least within a single tap.

Possibly incorporate FFTW library to get this to work.

[jackiryan]

This needs to be deferred as it has issues but is low priority.