Cleaned up docs and comments.  Marked PowerSpectrum and Spectrogram abstract for now, as they are not useful in the GUI.  Provided a more useful default for PowerSpectrum.window_length.  Removed default value for the signal argument, as it is not currently usable with default values anyway.  Added sanity check on window size.
Cleaned up docs and comments.  Marked PowerSpectrum and Spectrogram abstract for now, as they are not useful in the GUI.  Provided a more useful default for PowerSpectrum.window_length.  Removed default value for the signal argument, as it is not currently usable with default values anyway.  Added sanity check on window size.

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