walls and floor off by default
Use default wall thickness of 1.0 if --walls 0 (or not given)
This permits valid walls to be generated, even though disabled, so they can be enabled by changing the walls_enabled flag in openscad. Otherwise, 0 wall thickness borks wall geometry calculations
Separate thickness and enabled options for floor/walls
Floor and wall thickness is now set by --floor-thickness and --wall-thickness, respectively, with minimum values of 1
Whether the floor and walls are enabled by default are controlled by the --floor and --walls options (which now take no arguments)
floor_thickness is still passed to openscad, but now set in the "Hidden" section, so it's not visible to Thingiverse Customizer. No particular reason for this other than symmetry with wall thickness, which isn't extrudable in customizer at all.