Implements the filter Y{n} =a Y{n-1} + b Y{n-2} + d Y^2{n-l} + X{n} - c described in Dobson and sigmoid clipping. This filter is inherently unstable, so pick some sane values and move slowly from there.
Y{n} =a Y{n-1} + b Y{n-2} + d Y^2{n-l} + X{n} - c