fix mesh merge exception

[andrewheumann]

BACKGROUND:

• Adding the first vertex to a mesh with "merge" on would throw an exception.

DESCRIPTION:

• Fixes the exception.

TESTING:
-Added a new test which failed and now passes.

REQUIRED:

• All changes are up to date in CHANGELOG.md.

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This change is 

[wynged]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @andrewheumann)

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Elements/src/Geometry/Mesh.cs line 351 at r1 (raw file):

            {
                // if we've never added any vertices, we may not have a valid bbox, so we have to guess at a default size.
                var bbox = _bbox.IsValid() ? _bbox : new BBox3((-10, -10, -10), (10, 10, 10));

that's a big default box, but approved!