Updated Gerber Layer Format Specification

Ucamco has published the 2021.04 revision of the Gerber Layer Format Specification. 

We fixed errors and typos in the 2021.02 revision, pointed out by Bruce McKibben and Jim J. Jewett - we thank them for the very careful proof reading. The definition of the coordinate system was added to 4.2, as suggested by Graham Wideman, who helpfully supplied the illustrations.

You will find the full specification here.