Thumbwheel for Geared Heart

I love the Geared Heart modified by gzumwalt here: https://www.thingiverse.com/thing:253934 but wanted something more sturdy and less complicated. So I swapped out the hand crank and gears for a sturdy thumbwheel. This also improves the alignment of the axle shaft in gzumwalt's design, keeps it from having so much play and makes the heart run more smoothly.