Avoid loading multiple times the same data

There was a bug in `data.py` which made it load the same data as many
times as the number of postal codes in constraints. This is now fixed.

flatisfy/data.py

@@ -83,8 +83,13 @@ def load_data(model, constraint, config):
     get_session = database.init_db(config["database"], config["search_index"])
     results = []
     with get_session() as session:
+        areas = []
+        # Get areas to fetch from, using postal codes
         for postal_code in constraint["postal_codes"]:
-            area = data_files.french_postal_codes_to_iso_3166(postal_code)
+            areas.append(data_files.french_postal_codes_to_iso_3166(postal_code))
+        # Load data for each area
+        areas = list(set(areas))
+        for area in areas:
             results.extend(
                 session.query(model)
                 .filter(model.area == area).all()