plotter: removed unused import
plotter: renamed core attributes

    - BasicPlot._figure -> BasicPlot.figure
    - BasicPlot._ax -> BasicPlot.axes
Spectrum: allow reading from dat/txt file

    - added I/O registry for .dat or .txt files reading arbitrary
      columnar data into a spectrum::

          >>> from gwpy.spectrum import Spectrum
          >>> Spectrum.read('mydata.txt')

    - fixes #4