Quadrature mirror filter

[tlecomte]

In the current code, the octave and fractional-octave filters are implemented with a cascade of decimations and a set of bandpass filters for each octave, but only the low-pass signal is decimated. According to the nyquist theorem, the high frequency parts can also be high-pass filtered and decimated before the octave filters are applied. Quadrature mirror filters allow to do both the low-pass and high-pass decimation with one filter only. So the expected performance should be twice as high as the current implementation.