Provide frequency/duty_cycle API for PwmOut::begin(...).

[aentinger]

This fixes #168 .

A short example sketch to test this could be:

#include "pwm.h"

PwmOut pwm_freq(3);

void setup()
{
  Serial.begin(9600);
  while (!Serial) { }
  
  if (!pwm_freq.begin(10*1000*1000.0f, 50.0f))
    Serial.println("Error");
}

void loop() {

}

1 MHz ✔️

5 MHz ✔️

10 MHz ✔️

Although the generated frequency wanders ever more off the desired target. FYI @maidnl .