Allows code to explicitly specify which waveform generator it wants,
without needing to use one of the 100 IDE menus or adding a `-D`
compile-time define.

Uses weakrefs to allow for apps to call `enablePhaseLockedWaveform();`
within their `setup()` (or anywhere, really) and have the phase locked
versions override the default waveform generators automatically.

For example:

````
void setup() {
  // Uncomment following line to use phase-locked waveform generator
  // enablePhaseLockedWaveform();
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);     // Initialize the LED_BUILTIN pin as an output
  analogWriteRange(1000);
}
void loop() {
  analogWrite(LED_BUILTIN, 100);
  delay(1000);                      // Wait for a second
  analogWrite(LED_BUILTIN, 900);
  delay(2000);                      // Wait for two seconds (to demonstrate the active low LED)
}
````

Also adds an example showing it's use.

Address @dok-net's comments and also remove the _weak/_bound version of
startWaveform() since it's invariant of the actual waveform generator.