Add smoothPSD operator

[jdpigeon]

Smoothed PSD buffers are quite a bit better nicer to look at and work with than raw PSDs. For spectral analysis, they can be more accurate too as they're less sensitive to noise.

Once we have a solid PSD operator implemented, adding a smoothed version shouldn't be too hard. All we really have to do is collect a list of the last few PSDs and average them (i.e. reduce) to a single array