Polyphase decimation/interpolation filters

[astewart-xmos]

Implement polyphase FIR filters for decimation and interpolation.

These will be needed for updating lib_mic_array to take advantage of XS3

[astewart-xmos]

When investigating polyphase decimators, I realized that I'm pretty sure the existing FIR filter functions are suitable for polyphase decimation. To do decimation, with a decimation factor of K, when the nth input sample is received, compute z = n mod K, and if z == 0, call xs3_filter_fir_s32(), otherwise call xs3_filter_fir_s32_add_sample().

update:

int32_t poly_decimate(xs3_filter_fir_s32_t* filter, int32_t samples_in[K]){
  for(int k = 0; k < (K-1); k++)
    xs3_filter_fir_s32_add_sample(filter, samples_in[k]);
  return xs3_filter_fir_s32(filter, samples_in[K-1]);
}

[astewart-xmos]

Similarly, polyphase interpolation by a factor K can be performed as

void poly_interpolate(int32_t* output, xs3_filter_fir_s32_t* filter, int32_t sample){
  output[0] = xs3_filter_fir_s32(filter, sample);
  for(int k = 1; k < K; k++)
    output[k] = xs3_filter_fir_s32(filter, 0);
}

[astewart-xmos]

Given that decimation/interpolation are already possible by combining operations already available, I don't see any urgent need to add additional functions explicitly for decimation/interpolation.

This issue can be reopened later if it's deemed necessary.