def find_face_neighbors(face)
      raise "not a face" unless face.is_a?(Face)
      neighbors = find_neighbors(face.vertices)
      neighbors - [face]
    end
def find_face_neighbors(face)
      raise "not a face" unless face.is_a?(Face)
      neighbors = find_neighbors(face.vertices)
      neighbors - [face]
    end
Return all neighbors of the given faces
 Neighbor is defined as sharing a vertex,
 not necessarily sharing an edge.