A note can be placed in the center (panning=0) or moved to just the left
(panning=1) or right (panning=-1) channels. Fractional panning values
place it partially in both channels.
Now the vibrato 'units' are 1.0 = one octave, 1/12 = one semitone,
1/1200 = one cent. Before, the units were somewhat arbitrary and were not
perceptually "symmetrical" around the base frequency.
For vibrato_depth = 1/12 and base frequency of 440,
before: pitch from 403.33 to 476.67Hz, not corresponding to any notes
after: pitch from 415.30 to 466.16Hz, corresponding to G# and A#
This class allows much more expressive sound synthesis:
* tremolo & vibrato
* arbitrary frequency
* different evelope & waveform per note
* all properties dynamically settable from Python code
