Printer tests for deltas

This is a set of very basic tests of the mechanics of your printer. The intention is that they should be fairly slicer-independent, and they should each, as far as possible, isolate a single feature of printer performance.

XYZ size: measure the size of a print in line with the X Y and Z towers. Changes in these sizes are a first-order effect of miscalibrated arm lengths. Each size can be measured externally, as the size of an object, or internally, as the size of a void, so that over/under ex