Changes in models.py (GSystem class)


**Functionalities implemented:**

**NOTE - Run following command: ```python manage.py sync_existing_documents```**

1) New fields added to 'GSystem' class

    1) 'attribute_set'
        - data_type: [dict]
        - key: name of the AttributeType
        - value: value in object_value field of corresponding GAttribute

    2) 'relation_set'
        - data_type: [dict]
        - key: name of the RelationType
        - value: ObjectId in right_subject field of corresponding GRelation
      
    - TODO: I'm going to create generic functions that will update these fields as and when corresponding GAttribute(s)/GRelation(s) are created and/or updated!
   
    - Files modified:
   
      1) models.py
 
          - Above Mentioned Fields added under GSystem class
        
      2) management/commands/sync_existing_documents.py
     
          - Script updated to add 'attribute_set' & 'relation_set' fields with default values (as []) in all existing GSystem documents.