add OpenSCAD implementation

[schiele]

This model attempts to stay as close to the original as possible.
There are the following minor differences though:

• Cylinders and other roundings are split in a higher number of
  segments. The precision can be controlled by the $fa and $fs variables.
• Some incorrect labeling on the tolerance test have been corrected.
• The positioning of the text on the overhang tests was slightly adjusted
  since the original text was not positioned in a consistent way.
• The fillet at the achor point of the overhead test is designed in a
  slightly different way since this is easier to describe in OpenSCAD and
  also feels more consistent.
• The "OS" text has been added to the model version to clearly mark this
  not to be the original model.

All of those changes should not have any significant impact on the tests.
If you want to compare the results with existing results of other printers
you should use the original model though to be on the safe side.

This model was designed for easier integration with OpenSCAD and to be used
in a more programmatic context. Maintaining and adapting sources with
slightly more than 100 lines of code also seems easier than doing this with
a 2.2MB compressed binary file we need to open in a proprietary software
and therefore no easy way to version control the changes.