code: def unmerge_vertices(self):
          
          # new faces are incrementing so every vertex is unique
          faces = np.arange(len(self.faces) * 3,
                            dtype=np.int64).reshape((-1, 3))
  
          # use update_vertices to apply mask to
          # all properties that are per-vertex
          self.update_vertices(self.faces.reshape(-1))
          # set faces to incrementing indexes
          self.faces = faces
          # keep face normals as the haven't changed
          self._cache.clear(exclude=['face_normals'])
docs: Removes all face references so that every face contains
          three unique vertex indices and no faces are adjacent.