i3 Magnetic dual hotend mount

This is a design for a dual extruder with a simple magnetic coupling, inspired by OpenCircuitDesign's Makergear M2 twin extruder. Essentially, there are 3 parts to it: a middle block to which the belt is affixed and a left and right block that carry the hotends (E3D v6s). The middle block is spaced so that it cannot attach to both the left and right blocks at once, so through some G-code manipulation it's able to detach and reattach to either block without any extra electronics. This allows for