Add a real time spectrogram waterfall display

[rabssm]

Add a display of the spectrogram as a waterfall display similar to that displayed by Spectrum Lab.
This could be either a display feature added to the acquisition software, or a waterfall display available to a web browser, similar to
http://arc.id.au/Spectrogram.html

[rabssm]

Added --waterfall command line option to display a real time spectrogram waterfall.