code: def get_cell_length(flow_model):
          
          assert flow_model.lower() in FlowModelConst.d8_lens
          return FlowModelConst.d8_lens.get(flow_model.lower())
docs: Get flow direction induced cell length dict.
          Args:
              flow_model: Currently, "TauDEM", "ArcGIS", and "Whitebox" are supported.