Parametric Annular Fan duct for centrifugal blowers

Parts fan for my kossel.
I had a fan and I didn't feel like buying the one that fit the previous version so I made a parametric version... Pardon my coding.
Changed a thing or 2 to try and improve printability, extended the tab for pivoting as well.
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Changed mounting point for fan to be controllable rather than some magic number derived from strange math relationships.
changed screw size math so you specify diameter not radius as previously.

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initial tab math was bad, updat