Use a spherical or ellipsoidal Voronoi diagram

[toph-allen]

Voronoms uses straight lines drawn in unprojected longitude-latitude space to create its shapes.

This isn't as bad of an issues as it might at first seem, because when it's working correctly, most of the spatial structure comes from the layout of the underlying items. But the Voronoi diagram should be drawn on an ellipse.

Voronoms currently uses [scipy.spatial.Voronoi](https://docs.scipy.org/doc/scipy/reference/generated/scipy.spatial.Voronoi.html) to draw its Voronoi diagram. There's also [scipy.spatial.SphericalVoronoi](https://docs.scipy.org/doc/scipy/reference/generated/scipy.spatial.SphericalVoronoi.html), which at first seems like it does what I'd want. But it actually works really differently from the regular Voronoi module. You specify your points in three-dimensional space, along with the center and radius of the sphere you want to draw the Voronoi diagram on.

To use that, we'd need to convert from lon-lat to points on the surface of a sphere, which… I'm not sure the best way to do that right now.