Wheel improvements

Add a thicker central hub to the wheel to improve the connection to the stepper motor.
New Flexy Filament Tyre
if you move the render() call up to above the difference on line 156 it'll fix the rendering error - OpenSCAD throw-together view doesn't cope well with booleans on extruded stuff - it generally screws it up, so you need to move the render statement above the boolean that is having the problem