Accurate calibration/dimension test

####There is an new version of this: https://www.thingiverse.com/thing:2484766

This test jig is designed to analyse the dimensional accuracy of a printer using engineering methods. But don't be afraid: an excel sheet is included which performs all the math. You just have to print, measure and type in the numbers. A caliper is needed.

Note that calibration is normally not needed for modern printers and calibration cubes are very coarse. Check: https://www.youtube.com/watch?v=Mbn1ckR86Z8

Why this is more accurate than other calibration/testing methods:
- The edges have chamfers to eliminate effects from the first and last layers.
- Corners are rounded to minimize effects caused by sudden direction change.
- Different than a calibration cube the excel sheet does analyze the diffence in the length of the colums instead of their total lenght. This reduces the influence some effects wich would distort the calibration like a effective thicker wall due to z-wobble or over extrusi