improve release behavior.
This slows down the release after events like single snare hits as, the gain
reduction only releases to 160 dB rather than to inf.
To properly visualize attack and release we calculate the input level out of
the gain reduction the output level threshold and ratio.
if (output_level >= threshold) { // no expansion
input_level = output_level
} else {
input_level = (gain_reduction - threshold * (1-ratio)) / ratio
}
