MAINT: circular SphericalVoronoi input

[tylerjereddy]

As noted by @pmla:

[I]t has occurred to me that the Voronoi vertices for circular input are incorrect. Circular input has exactly two Voronoi vertices - one at each pole. The other "vertices" I have calculated (those in between each pair of adjacent input points) make it possible to plot regions correctly, but are not actually Voronoi vertices. If we use the actual Voronoi vertices only, we need a different mechanism for region plotting. This could be e.g. representing regions as plane normals, but this would be a big change. Otherwise we could add a note to explain that only two vertices are actual Voronoi vertices, and the rest are merely "helper" vertices for plotting.

Since circular input has provided an error or garbage in previous releases, what we're doing now is clearly better. There's also an absence of literature guidance on this scenario as far as I can tell.

I'd suggest removing the "guide vertices" between generators and document the use of i.e. geometric_slerp to achieve reasonable plots without those "guides."

We don't actually offer plotting functionality, so just documenting the suggestion seems reasonable.

One possible point of hesitation to using just two Voronoi vertices here--would sort_vertices_of_regions() handle this gracefully?

[pmla]

One possible point of hesitation to using just two Voronoi vertices here--would sort_vertices_of_regions() handle this gracefully?

It will raise an exception:

        for i in range(3):
            k = simplices[current_simplex, i]

The 'simplex' contains only two vertices. This is not actually a simplex - we can't represent the Voronoi regions as a collection of points. The Voronoi regions of all input points contain the same two Voronoi vertices.