docs: fixed BodePlot example to be made on-the-fly
docs: updated unit on ASD plot for index
Spectrum: added epoch to from_lal
TimeSeries: minor metadata property edits
TimeSeries: fixed type set in span property
Array: added epoch and channel properties

    - all data arrays should be derived from some instrumental signal
    - all data arrays should be associated with a GPS time (epoch)
Array: minor edits to __repr__

    - __repr__ will try to access all _metadata_slots properties, and
      will fail if they are not set, or don't have a None default return
Array: removed unit setting in __array_wrap

    - added __pow__ and __ipow__ methods to handle powers and set the
      units appropriately
Array: bug fix to __ipow__
TimeSeries: bug fix in asd/psd

    - don't assume a TimeSeries has a unit
Array: bug fix in __ipow__
Array: remove print statement in __pow__
Series: bug fix
Array2D: minor edit and bug fix
segments: modified segments to sub-class GLUE

    - need sub-class to facilitate extra features in segments
docs: remove non-existent page reference
docs: remove non-existent method reference
docs: removed non-existent page reference
Spectrum: fix bug and pep-8 syntax
examples: plot spectrogram

    - updated docs
    - updated example figure display/save