Enable automated window size determination of Savitzky-Golay and Mean filter (maybe also Median filter)

[MothNik]

🚶➡️🏃 Proposed Enhancement

Today, I answered this StackExchange Question on the Savitzky-Golay filter and thereby I figured out that the automated selection of the Savitzky-Golay (and thus the mean filter because it is the Savitzky-Golay filter with polynomial degree 0 😱) is easily possible via Cross Validation.
After the WhittakerSmooth might get a facelift (#120) with that enables automated smoothing, I think this is a natural way to proceed. If we trick a bit, actually also the median filter can get this update, but the numerics might be a bit more involved.

[MothNik]

Basically, this would also allow for outlier detection in a next step.