Improved customizable fan duct

I made a small improvement to the original version of this duct generator, by adding a parameter that allows you to specify the length of the "throat" of the duct. That is, the section with a straight inner wall where the fan blades come closest to the duct wall. On a single fan duct, this can be zero, but for contra-rotating fans, such as I intend to use, this is approximately the vertical spacing between the 2 fans, maybe with a tiny fudge factor thrown in. This is specified in mm.