Customizable Hirth Joint (vertex version) (library)

**There's a greatly improved version:
"Customizable Hith Joint (CSG version) (library)"**
http://www.thingiverse.com/thing:397553
.
This is a trapezoidal Hirt-Joint generator.
usage:
* either use customizer
* or use it as a library:

hirth(
  period_number,
  flank_angle,
  base_hight,
  vertex_radius,
  [cutround_diameter|default=0...do_not_cut_round],
  [bore_radius|default=0...no_bore],
  [vertical_stretching_factor|d