“YOU can carry your own head in your hand,” enthuses Bre Pettis, inviting customers to try out a three-dimensional photo booth that will scan their head and then print a miniature plastic version of it as a solid object. This is useful, no doubt, for those about to audition for the role of Zaphod Beeblebrox in “The Hitchhiker’s Guide to the Galaxy”. Mr Pettis, the founder of MakerBot, a maker of l