MAGNET_WALL_THICKNESS not honored in "sheath"

[ivanpointer]

Setting MAGNET_WALL_THICKNESS changes the body, but in the sheath, the value is hard-coded, creating issues with snapping the sheath and body together when this value is changed - especially decreased.

The solution is to simply specify this value on the sheath in void_switch.scad - the function already has a parameter for it, but because it's not passed, the function uses the hard-coded default of 0.5.

I'll be submitting a PR for this shortly.

[ivanpointer]

Submitted PR #20.