Pyembroidery

Adding in additional elements for some builtins being used in EmbPattern, EmbMatrix, EmbThread. Added styloInt's merging patterns functionality (60). You can now technically add patterns together. `pattern_merged =pattern1 + pattern2` or `pattern1 += pattern2`. And implemented what it means for those objects to be equal or not equal.

Slight tweaks to the gcode reader to handle alternative comment formatting, and to process g20, g21 commands.

Bunch of improvements to the .xxx writing format. It seems to be consistent with what most writers are doing. There's no known ground where a critical flaw could exist that wouldn't also exist on many other programs. It should stitch.

Adds an encoder `round` parameter which turns out to be needed in a very limited set of circumstances.

Adds in experimental .xxx writer.

Adds in basic functionality for a gcode writer and reader.

It was requested to duplicate the Embroidermodder txt functionality.