These function compute the Dirichlet-Voronoi tessellation of X
(see dirichlet)
and extract the vertices or edges of the tiles of the tessellation.
The Dirichlet vertices are the spatial locations which are locally
farthest away from X, that is, where the distance function
of X reaches a local maximum.
The Dirichlet edges are the dividing lines equally distant between
a pair of points of X.
The Dirichlet tessellation of X is computed
using dirichlet. The vertices or edges of all
tiles of the tessellation are extracted.
For dirichletVertices, any vertex which
lies on the boundary of the window of X is deleted.
The remaining vertices are returned, as a point pattern,
without duplicated entries.