Time-frequency Automatic Gain Control (AGC)

This code implements automatic gain control for audio signals, which attempts to apply a smoothly-varying gain to an audio waveform in order to keep its energy close to some target level. This version also tries to balance the energy in individual frequency bands. It works by taking the short-time Fourier transform of the signal, smoothing that energy using time and frequency scales specified as arguments, then dividing out that scaled energy.