renaming `Affine.Simplex.points`

Stream: mathlib4

[Jovan Gerbscheid]

I find the name Affine.Simplex.points confusing, e.g. in s.signedDist i (s.points j) = 0, s.points j is just one point, not multiple points. So I'd prefer it to be the singular point or vertex.

Furthermore, it would be even more convenient if there was a CoeFun instance, so we could just write s.signedDist i (s j) = 0 instead.