code: def _tarboton_slopes_directions(self, data, dX, dY):
          
  
          return _tarboton_slopes_directions(data, dX, dY,
                                             self.facets, self.ang_adj)
docs: Calculate the slopes and directions based on the 8 sections from
          Tarboton http://www.neng.usu.edu/cee/faculty/dtarb/96wr03137.pdf