Deprecate dutyCycle() in favor of width() for SquareOsc

[sofian]

Function width() for TriOsc would be also appropriate as a replacement of dutyCycle() for SquareOsc and would contribute to a better uniformity in the library. It would resonate perfectly well with the concept of "pulse-width" (as in pulse-width modulation / PWM).

The visual documentation should be updated as well.