adding the spatial lag function

src/pg/sql/21_spatial_lag.sql

@@ -7,11 +7,10 @@ CREATE OR REPLACE FUNCTION
       num_ngbrs INT,
       geom_col TEXT,
       id_col TEXT)
-RETURNS TABLE (lag NUMERIC, rowid INT)
+RETURNS TABLE (spatial_lag NUMERIC, rowid INT)
 AS $$
   from crankshaft.spatial_lag import Spatial
   spatial = Spatial()
-  # TODO: use named parameters or a dictionary
   return spatial.spatial_lag(subquery, column_name, w_type,
                           num_ngbrs, geom_col, id_col)
 $$ LANGUAGE plpythonu;

src/py/crankshaft/crankshaft/spatial_lag/spatial_lag.py

@@ -41,6 +41,6 @@ class Spatial:
 
         # calculate spatial_lag values
 
-        lag = ps.weights.spatial_lag.lag_spatial(weight, attr_vals)
+        spatial_lag = ps.weights.spatial_lag.lag_spatial(weight, attr_vals)
 
-        return zip(lag, weight.id_order)
+        return zip(spatial_lag, weight.id_order)