Universal Ball Joint - OpenSCAD

Universal ball joint.

This is intended to be used as a library in other projects.
Example:
```
use ;
balljoint_ball();
balljoint_seat();
balljoint_nut();
```

If you need a smaller or larger ball joint, use "scale".
Example:
```
use ;
scale(2) balljoint_ball();
scale(2) balljoint_seat();
scale(2) balljoint_nut();
```

The ball joint was tested using the defined dimensions. Changing the dimensions may generate disfunctional parts. If you need to change the dimensions, import the library using "include" instead of "use" and overwrite the appropriate variables.
Example:
```
include ;
balljoint_ball_dia = 30;
balljoint_ball();
balljoint_seat();
balljoint_nut();
```

Inspired from:
- Universal Ball Joint by MORONator: https://www.thingiverse.com/thing:5020323

Dependency:
- helix_extrude.scad: https://www.thingiverse.com/thing:2200395

Unit: millimetres.