oppositeFaceDistance (edge) {
    return edge.face.distanceToPlane(edge.opposite.face.centroid)
  }
Computes the distance from `edge` opposite face's centroid to
`edge.face`

@param {HalfEdge} edge
@return {number}
- A positive number when the centroid of the opposite face is above the
  face i.e. when the faces are concave
- A negative number when the centroid of the opposite face is below the
  face i.e. when the faces are convex