This enables the specific use case of checking whether a note's release
phase has ended, but is also potentially useful to implement a sort of
"voice stealing" algorithm in Python code, which can take account of
the note's envelope state as well as other factors specific to the
program.
The underlying routine can return numbers for higher and lower
octaves. Other bits of the code might have frequency limitations
but that doesn't mean we shouldn't let someone get a ~4Hz "note"
by sending in (-12), because that's actually totally plausible as
an LFO frequency.
Semi-incompatible name change: The method `release_then_press`
is now `change`. For now a compatibility alias is supported.
Everywhere a `NoteSequence` was accepted, a single note is now accepted.
So for instance, `synth.press(30)` can be written instead of requiring
``synth.press((30,))`. The same goes for `change.retrigger`, which
will accept a single LFO or a sequence.
When there's no sustain, the release step needs to be calculated from
the attack level, not the sustain level. Otherwise, contrary to intent,
this leads to the actual release taking a loooonnngg time.
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#
