allow specifying the target frequency while detecting

[nonchip]

since the constructor doesn't seem to have to prepare much except for do a single formula to get the coeff value, i don't see the need to attach a fixed target frequency to the instance.

currently one would have to instantiate multiple objects and have them each reach the analog pin N times to e.g. distinguish between multiple frequencies, something that'll slow down the effective sample rate.

[nonchip]

untested, but this seems to be a 1-function version of this algo achieving this:

#define ADC_MIDPOINT 512
#if F_CPU == 16000000L
  #define SAMPLE_FREQ 8900
#else
  #define SAMPLE_FREQ 4400
#endif

float goertzel(int samples[], int num_samples, float freq){
  float Q1 = 0;
  float Q2 = 0;
  float coeff = 2.0 * cos((2.0 * PI * freq) / SAMPLE_FREQ);
  for (int i = 0; i < num_samples; i++) {
    float Q0 = coeff * Q1 - Q2 + (float) (samples[i] - ADC_MIDPOINT);
    Q2 = Q1;
    Q1 = Q0;
  }
  return sqrt(Q1*Q1 + Q2*Q2 - coeff*Q1*Q2);
}