suggestion: add abstraction to calculate biquad coefficients

[porres]

I wonder if we could have an object like else/biquad. All it does is implement some basic filters from the "Eq-cookbook". This is a compiled object but I also have a vanilla abstraction. Maybe such a vanilla abstraction could be included in the 'extra' folder as an extra object. This would make [biquad~] much more useful for the general public allowing Pd to have many built in 2nd order filters, namely: allpass, lowpass, highpass, bandpass, resonant, bandstop, eq, lowshelf, and highshelf (lowpass and highpass being resonant).