Splines for OpenSCAD

I was working on a project in OpenSCAD that needed smooth curves. I was excited to find some [Bezier curve libraries](http://www.thingiverse.com/thing:8443), but as I used them I remembered why awkward thing about Bezier curves: they're hard to design with if you want precise control. You move a control point, and it pulls the curve one way, but it also affects other areas of the curve, so you have to nudge adjacent control points the other way to compensate, and then repeat forever.

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