support mel scaling

[swharden]

add a method to Spectrogram which returns a mel-scaled 2D array

[swharden]

wrote some working code, but still not sure where to put it

Mel Scaled Data

Linearly Scaled Data

[swharden]

You can now create a Mel spectrogram like this:

Bitmap bmp = spec.GetBitmapMel(melSizePoints: 250, intensity: 4, dB: true);
bmp.Save("halMel.png", ImageFormat.Png);

Cropped Linear Scale (0-1kHz)	Full Linear Scale (0-22 kHz)	Mel Scale (0-22 kHz)