Moved 3d printing stuff to the correct location

slides/3dprinting.html

slides/img/3dprinting/add-subtract.png

slides/img/3dprinting/additive.png

slides/img/3dprinting/biotoxicity.jpg

slides/img/3dprinting/bridge-overhang.png

slides/img/3dprinting/bridge.png

slides/img/3dprinting/bust.png

slides/img/3dprinting/cube-shell.png

slides/img/3dprinting/cube-stl.png

slides/img/3dprinting/embedding.png

slides/img/3dprinting/fabric.png

slides/img/3dprinting/fabric2.png

slides/img/3dprinting/gear.png

slides/img/3dprinting/grey-box.png

slides/img/3dprinting/interactiles.png

slides/img/3dprinting/ioio.jpg

slides/img/3dprinting/magician.png

slides/img/3dprinting/metal-embed.jpg

slides/img/3dprinting/nutbolt.png

slides/img/3dprinting/overhang.png