group_set field moved from GSystem class to Node class


- Files modified:

  1) models
     - group_set field moved from GSystem class to Node class
     
  2) add_fields_to_existing_documents
     - Adds group_set fields into already exsisting documents; except in GAttribute, GRelation documents, and documents where it's already exists.
     
- Run following command:

  ```
  python manage.py add_fields_to_existing_documents
  ```