radius gauge with minor tweaks

This is a slightly tweaked version of [Grant Stevens](https://www.thingiverse.com/grantstevens/about)’ [original](https://www.thingiverse.com/thing:910616) radius gauge.

Touch the tool's 3 disks to a curved surface, and read the radius on the dial.

I think the original idea and design were brilliant. That doesn’t mean that there wasn’t room for small improvements, i:
* used a nicer looking font, because, ugh.
* added the unit (cm only, no multiples of [0.9144 m](https://en.wikipedia.org/