Makerfarm i3v固定Z螺母固定

Re-designed the Makerfarm i3v wooden Z nut holding piece to actually keep the nut captive, and in doing so also reduce backlash due to the poor tolerances of the laser cut wood design. Piece is a drop in replacement with no additional hardware but due to the captive design does require removing either the threaded rod from the motor attachment to slide it on from the bottom or removing the nut entirely from the threaded rod. Nut should be a nice and comfortable press fit. Piece is overall smaller than the wooden part it is replacing and should have no clearance issues.