python/demos/demo_specdesc.py: remove unused import
python/demos: python3 and double precision compatibility
python/demos/: minor plot improvements
python/demos/demo_{mel-energy,mfcc,specdesc}.py: use set_xlabels_sample2time
python/demos: add demo_pitch.py and demo_waveform_plot.py
python/tests/test_fft.py: update, indent