Kossel Magnetic Effector

This is my WIP Kossel effector for magnetic arms with ball-studs and an E3D v6. It also features a Y-joint built into the effector mounting bracket to allow for single nozzle dual extrusion.

Note that while the preview screenshot contains a fan, I haven't finished that mount yet.

Here's the gcode I use for tool change in Simplify 3d:
```
;BEGIN TOOL CHANGE
G92 E0
G1 E-9 F[toolchange_retract_speed] ; retract previous tool
G91
G1 Z5 F[travel_speed] ; lift
G90
G1 E-40 F300
G1 E-[tool