Refactor and new tests

pg/sql/0.0.1/03_overlap_sum.sql

@@ -0,0 +1,20 @@
+-- Function by Stuart Lynn for a simple interpolation of a value
+-- from a polygon table over an arbitrary polygon
+-- (weighted by the area proportion overlapped)
+CREATE OR REPLACE
+FUNCTION cdb_overlap_sum(geom geometry, target_table_name text, target_column text)
+  RETURNS numeric AS
+$$
+DECLARE
+	result numeric;
+BEGIN
+  EXECUTE Format('
+    SELECT sum(%I*ST_Area(St_Intersection($1, a.the_geom))/ST_Area(a.the_geom))
+    FROM %I AS a
+    WHERE $1 && a.the_geom
+  ', target_column, target_table_name)
+  USING geom
+  INTO result;
+  RETURN result;
+END;
+$$ LANGUAGE plpgsql;