Separate the user configuration into a separate file.
Remove code defining A and B variants.
Drop A and B variants from Makefile.
Change documentation to remove A and B variants.

Removed the main documentation about the A and B variants. Added a
little blurb stating that they existed, but are no longer supported.
That way I don't need to rename C and D, yet.
Attempt to remove attempts to merge STLs.

The .gitattributes file tells git to keep our version of all STL files
rather than try to merge them. This should reduce spurious conflicts
during merges. There's really no reasonable way to correct the
conflicts.

We are keeping the STLs in version control despite being generated files
because it saves people from needing to use OpenSCAD to generate them
and they are viewable on github.
Merge branch 'remove_variants'
Regen STLs after variant cleanup.

Remove the A and B variants.
Regenerate the C an D variants.