Filter implementation with cascade of biquads

[tlecomte]

Currently, high-order IIR filters are used for decimation and fractional octave filters, and they are implemented as brute-force filtering, whereas it is advised to use series of biquads to achieve more stability (the use of floats instead of integers mitigates the issue).

[endolith]

This will be built into scipy soon: scipy/scipy#2444

[tlecomte]

@endolith: Thanks !! It will be interesting to use that when released. If only I had more time to test it...