def triangle(index)
      return nil if( index < 0 || @tri_indices.size <= index )
      tri_index = @tri_indices[index]
      return Triangle.new(vertices[tri_index[0]], vertices[tri_index[1]], vertices[tri_index[2]])
    end
def triangle(index)
      return nil if( index < 0 || @tri_indices.size <= index )
      tri_index = @tri_indices[index]
      return Triangle.new(vertices[tri_index[0]], vertices[tri_index[1]], vertices[tri_index[2]])
    end
[Input]
  _index_ is index of triangle.
 [Output]
  return new instance of Triangle.