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-[toolchange_retract_distance] F[toolchange_retract_speed]
T[new_tool] ; switch to new tool and offsets
G1 X[next_position_x] Y[next_position_y] F[travel_speed] ; move to next printing location
G92 E0
G1 E[toolchange_prime_distance] F1800 ; prime new tool
G92 E0
G1 Z[next_position_z] F[travel_speed]
;END TOOL CHANGE
```

Note that this absolutely requires a priming tower.
The key insight in the gcode is to do a normal retraction, a slow retraction to let the filament cool rather than making