DataQualityFlag.texname: fix AttributeError
SegmentPlot: fix name == None errors
gwpy.table: reworking for glue Table extensions

    - importing gwpy.table now annotates the glue.ligolw.table.Table and
      lsctables sub-classes with extra methods including

        - read() - unified I/O
        - event_rate(), binned_event_rates() - generate TimeSeries of
          rate (Hz) for event triggers
io.cache: correct moronic typo (that does nothing)
gwpy.table: minor improvements