Change cookiecutter base to use ElementsWorker

worker-{{cookiecutter.slug}}/worker_{{cookiecutter.slug}}/worker.py

 # -*- coding: utf-8 -*-
-from arkindex_worker.worker import BaseWorker
+from arkindex_worker.worker import ElementsWorker
 
 
-class Demo(BaseWorker):
+class Demo(ElementsWorker):
     def process_element(self, element):
         print("Demo processing element", element)