python/demos: python3 and double precision compatibility
python/demos/demo_spectrogram.py: remove unused import
python/demos/demo_spectrogram.py: fix title
python/demos/demo_spectrogram.py: fix indent
python/demos/demo_spectrogram.py: fix indent
demos/demo_spectrogram.py: add samplerate option to spectrogram
moved tests to subdirectory