spectrum (spec/04 §4.11): a time series to its one-sided amplitude spectrum by FFT
(Macaw.Data.FFT.spectrum/3). The interval is every, else the median positive gap of
the time column, else 1; the samples are the newest power of two rows with a numeric
reading of field; each row holds as's names for the frequency and the magnitude, and
a third, when as gives one, for the phase.