Allow detecting location availability per postcode row

If `--no-location` is not set we would try to detect a location for each row, and this would break if the location fields could not be coerced into floats.  Some datasets mix location and non-location postal codes and to import them all we have to filter the data and run the importer twice.

This change allows individual importers to implement `location_available_for_row` to say if the supplied row has location data or not.  The method is called on each row and will run the `--no-location` path if we can't extract location fields for that row.  If `--no-location` is set, we always run that path, regardless of the `location_available_for_row` value.

mapit/management/commands/mapit_import_postal_codes.py

@@ -109,8 +109,11 @@ def pre_row(self, row, options):
     def post_row(self, pc):
         return True
 
+    def location_available_for_row(self, row):
+        return True
+
     def handle_row(self, row, options):
-        if not options['location']:
+        if not options['location'] or not self.location_available_for_row(row):
             return self.do_postcode()
 
         if not options['coord-field-lon']: