add additive line height & width comments

[AndyEveritt]

Currently Simplify3D only displays a default additive line thickness and height (0.4 x 0.2 mm). This can make the preview hard to understand if using alternative layer heights

The viewer determines the bead size based off of a comment with the following format:

; tool H#.### W#.###

where the ";" is the comment character, the decimal number after the H represents the bead's height (mm), and the decimal number after the W represents the bead's width (mm).

Need to add this to the gcode Fusion makes (can probably be done in the post processor)