Shelf filter support for biquad

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I added support to configure biquad stages with shelving filter response: Link to GitHub.

The functions are
setLowShelf(stage, frequency, gain, slope);
setHighShelf(stage, frequency, gain, slope);

This picture describes what is what:
shelf filter.png
The frequency describes the slopes midpoint, the amplitude at this point is always (gain/2), independent of the slope parameter.
Gain is specified in dB and can be positive or negative.
The slope parameter configures how steep the change in amplitude will be, gain=1 is maximum steepness without overshoot/undershoot in the frequency response. gain < 1 is a less steep curve, gains > 1 have even steeper transitions at the specified frequency at the cost of over-/undershoot.

I added documentation to the Design Tool (GUI): Link
I tested both low and high shelf filters with various frequencies, gains and slopes and checked with the FFT function of my oscilloscope. The filters work as intended.
Hi Ben,

I decided to run each filter into another mixer and use the gain functions of the mixer. This is certainly not easy with a simple gain input of 1 to 1dB. I am curious is there documentation on the transfer function for the bandpass and notch/band stop filters?
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