Optimize DSP work on iOS using the native Accelerate framework

[tonelli-m]

DSP functions such as FFT, filtering, downsampling, etc could be optimized on iOS by using the native Accelerate framework.

Some useful links:

• FFT
• Filtering
• Downsampling
• There is even library code to generate spectrogram images

This would mean spending some extra time for developing platform specific implementations, but since it's just about using a library it should take less time than reimplementing algorithms. Thing is it is hard to tell what the performance benefit would be before trying it out.