#97 Support multilateration cases where all BS are on the same plane.

[Jesper W]

The current multilateration solver fails if all the BS are on the same plane. Not a problem if the BS are really Base Stations in the real world, but a bit limiting in other use cases, such as mine, where they are really microphones mounted on a small planar plate.

Maybe this use is best handled by a special 2D version of the solver?