Customizable single-twist Solomonic column

This is a simple application of my [tubemesh library](https://www.thingiverse.com/thing:2406760) for OpenSCAD.

You can also do this with linear_extrude(), but tubemesh would allow for more complex effects (e.g., bulging in the middle, etc.), and with optimize=5 or so, it will generate a better quality mesh.