Double-Decker MPCNC

Work in progress, and repository for some of the modified parts I'm creating in the process.

* XYZ_J_Burly_Label, no different from XYZ_J_Burly except that it has a label for tension bolt B
* XYZ_J_Burly_Label_Trimmed, similar to XYZ_J_Burly_Label but also cut away around the outermost bolts to make for easier assembly
* XY_J_Burly_Label, no different from XY_J_Burly except that it has labels for tension bolts A and C
* CornerBrace: experimental replacement for corners where the vertical leg can pass through. Now has slot for belt, but method for securing belt is not yet complete.
* C_Roller_With_J_Gantry: allows "mix-and-match" rails where legs and side rails are 3/4" EMT conduit while gantry and Z-axis is made from 25.4 mm tube.
* XYZ_J_Extra_Burly: same as XYZ_J_Burly but has extra material around the outer nuts to help me tighten them

Image is a sketch of the ultimate goal: blue and yellow rails are 1" tube, while green rails are 3/4" EMT. The hope is for it to be very