Make slots for optics mounts.

Also corrected the forehead_depth parameter, which I had mis-labeled.
Extended depth to match optics information from Dmitry.
Fix strap mount height.
Merge branch 'master' into optics_mount
Update STLs for current optics mount.
Correct merging error.
Merge branch 'master' into optics_mount
Design sliders to mount optics.
Better slide attempt and slot test.
A little Makefile cleanup.
Slider fits reasonably well.
First try at a lens holder.

Based on the dive design. Not any clips, yet. It will be hard to design
that part until I get the lenses in the mail.
Improve strength of the slider.
Clean up lens holder opening.
Attempt a better fit for slider.

Tighten the inside/outside fit, so that gluing is not required.
Tighten the slot fit so that it doesn't wiggle as much.
Add slope to the hole for the lens holder to make it parallel to the
phone.
Prepare for use of wall angle for optics.
Add documentation to the optics file.
Don't enable the angle code yet.
Tilt the slider in y not x.
Prepare main file to print whole visor.

Set up both A and B variants to print with the optics mounting hardware
on the same plate as the visor body.
Minor cleanup and add header comment.