Heavy Mendel, a self replicating 3D printer/mill/PCB engraver

The design is inspired by the triangular frame of the reprap Mendel, It will be heavier and more rigid to be able to mill plastic, engrave circuit board and print at high speed.

Differences compared to the standard Mendel :
- vertical X axis to quickly change the tool (extruder or spindle)
- M10 and M12 threaded rods instead of M8 for better frame stiffness
- 10mm smooth rods instead of 8mm for better axis stiffness
- Precision doubling belt configuration inspired by Madkite's design : ht