Archimedean spiral generator

[Archimedean spiral](https://en.wikipedia.org/wiki/Archimedean_spiral) is the base of [Spiral moving fish](https://www.thingiverse.com/thing:1763139).

The equation of Archimedean spiral is `r = a + b * theta`. If you just increase theta with the same amount, the distance between each segment of the moving fish would also be increased.

In order to equally spaced each segment, I need to find out all angles between each segment. That's what the `find_parent_angles` module does in the .scad file.

For more precisely controlling models between each segment, I also divide all parent angles found by the `find_parent_angles` module into several sub angles which may be controlled by the `children_per_step` parameter.

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- *[OpenSCAD Documentation - Traditional Chinese](http://openhome.cc/Gossip/OpenSCAD/)*
- *[OpenSCAD Documentation - English](http://openhome.cc/eGossip/OpenSCAD/)*

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