Retraction / Flow test with Cura & gcode files

I wanted a retraction tower that could test 20 retraction lengths, not use much material, and have normalized change heights for ease of use. The info below describes how to use this file to fine-tune retraction lengths.

**The goal is to start with the retraction lengths too short, then step them up incrementally until the stringing disappears. Hopefully, there will be enough tests to start seeing artifacts from over-retraction, which will look like missing bits of layer, holes, or pits in the wall surface. Then you can find the midpoint between problems.**

**Once you have a range (for example, 0.9-1.2 seem ok in the picture above), then you can again modify the tower to test every iteration of [0.9, 1.0, 1.1, 1.2]mm retraction and [40, 35, 30, 25, 20]mm/s retraction speed, a total of 4x5=20 tests.**

*You should have dialed-in extruder retraction in 2 tests/4g material/60 min. with this test*

Warning
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Don't get crazy with huge retraction numbers. If you're getting