OpenSCAD Hirth Joints

https://github.com/mechadense/scad-lib-hirthjoints

Author: Lukas M. Süss aka mechadense
License: LGPL-3.0
Description: A minimal function for generation of hirth joints.

This is more meant as a library.
Deliberately kept minimal.

Customizer (TODO needs debugging)
is just meant as a quick preview of what can be done.
Note that there is no math for sanity checking of supplied parameters.

## Usage tipps:

```
use
hirth
( n = 12
, r = 24
, zbase = 8
, slant = 60
, a1up = 10
, a1down = 12
, cuttype = "cylindrical"
, dbore = 12
, acone = 0
)
```

- Always keep down_angle bigger than up_angle or the roofs will hit the valleys.
- If you use ist as a cutter then increase r and decrease dbore compared to the counterpart such that the teeth will fit radially.
- For printing a hirth interface at the bottom of a FFF-3D-print one can avert the need for support material by cutting off half of the teeth. This still meshes with full teeth but