delaunay + voronoi

[mbostock]

Fixes #812. TODO:

• delaunayLink
• delaunayMesh
• hull
• voronoi (voronoiCell?)
• voronoiMesh
• pointerEvents
• faceting
• variable styles for delaunayLink (computed by target data)
• variable styles for voronoi
• group by z for voronoi
• group by z for delaunayLink
• group by z or stroke for delaunayMesh
• tests
• documentation

Plot.plot({
  marks: [
    Plot.delaunayLink(data, {x: "culmen_depth_mm", y: "culmen_length_mm", stroke: "culmen_length_mm"})
  ]
})

[mbostock]

Plot.plot({
  marks: [
    Plot.dot(penguins, {x: "culmen_depth_mm", y: "culmen_length_mm", fill: "currentColor", r: 1.5}),
    Plot.voronoi(penguins, {x: "culmen_depth_mm", y: "culmen_length_mm", stroke: "species"})
  ]
})

[Fil]

In the case of Delaunay, I can see how it is useful to connect points by group; I've tried to implement it in #925. However I can't really figure a use case for doing this with Voronoi (except to be consistent? Being consistent is a good thing.).

I'm curious also how you plan to use pointerEvents, probably with titles? Should we add titles in the penguinCulmenVoronoi test? A nit: commit e8ddf63 should probably be separated from this PR since it affects all marks.

[Fil]

🎉

Suggestions:

• add an image of Delaunay + hull?(test/output/penguinCulmenDelaunaySpecies.svg demonstrates it well).
• default pointerEvents to "auto" (equivalent to "visiblePainted", but more well-known?)

#928 tracks a few follow-ups that I have in mind.