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