code: def face_midpoints(self, simplices=None):
          
  
          if type(simplices) == type(None):
              simplices = self.simplices
  
          mids = self.points[simplices].mean(axis=1)
          mid_xpt, mid_ypt = mids[:,0], mids[:,1]
  
          return mid_xpt, mid_ypt
docs: Identify the centroid of every simplex in the triangulation. If an array of
          simplices is given then the centroids of only those simplices is returned.