fischertechnik Bow-shaped beam (Bogenstück) 15° and 7,5°

2022-11-16: I added a [fully parametric design](https://www.thingiverse.com/thing:5631756).

The [fischertechnik](https://www.fischertechnik.de/en) construction toy system has [bow-shaped beams (Bogenstücke) with 60° and 30°](https://ft-datenbank.de/tickets?fulltext=bogenst%C3%BCck) as part of their statics line. Combined with the [flat girder part (Flachträger)](https://ft-datenbank.de/tickets?fulltext=Flachtr%C3%A4ger) you can construct circular structures with a diameter of 22,9 cm (60°) an