code: def face_midpoints(self, simplices=None):
          
  
          if type(simplices) == type(None):
              simplices = self.simplices
  
          mids = self.points[simplices].mean(axis=1)
          mids /= np.linalg.norm(mids, axis=1).reshape(-1,1)
  
          midlons, midlats = xyz2lonlat(mids[:,0], mids[:,1], mids[:,2])
  
          return midlons, midlats
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.