Biquad: Bandpass filter

[rtoy]

A follow-up of #419, for the banpass filter.

Blink uses

alpha = sin(w0) / (2 * Q);
k = cos(w0);

b0 = alpha;
b1 = 0;
b2 = -alpha;
a0 = 1 + alpha;
a1 = -2 * k;
a2 = 1 - alpha;

This has exactly the same form as the cookbook, if alpha = a_BW. But alpha here is the same as the cookbook's a_Q, not a_BW.