plotter: moved segment plotting from TimeSeriesAxes to SegmentAxes

    - can't think of a use case for plotting segments directly onto a TimeSeries
    - fixed a bunch of minor warnings
SpectrumAxes: added method to plot shaded spectrum

    - plots 'mean' spectrum with 'min' and 'max' shading
Array2D: fixed a bunch of warnings, and allow empty data
plotter.timeseries: fixed bug and a bunch of warnings
TimeSeries: fixed a bunch of warnings
Spectrogram: fixed a bunch of warnings