3D Retraction Test Suite

**Important update 27-Jan-2020**

I have uploaded corrected speed test files. The distance test files were (and are) are OK. **Do not use speed tests downloaded prior to today**.

There were miscalculations in the speed test files, and I temporarily removed them on 25-Jan-2020. All had incorrect speeds and three had ridiculous retractions. I apologise for any problems this may have caused.

The files here are the products of my attempts to make a comprehensive set to _quickly_ determine **best retraction distance** and **retraction speed** for an FDM printer/filament combination. (I have a similar set of temperature tests elsewhere.)

Each file consists of a base to ensure bed adhesion, and two slim pillars 20mm apart. The pillars are vertically divided into labelled 5mm sections to make it clear where parameters change. They are angular, partly to print faster, but mainly to ensure the travel moves start and end in the same places.

Each takes about 20 minutes to pri