anchor.diff:                  },
                   "notification": {
                       "callback": "http://localhost:1234",
  +                    "headers": {
  +                      "X-MyHeader": "foo"
  +                    },
  +                    "query": {
  +                      "authToken": "bar"
  +                    },
  +                    "attrsFormat": "keyValues",
  +                    "dataFormat": "object",
                       "attributes": ["temperature", "humidity"],
                       "throttling": 5,
                       "count": 12,
  
positive.code: A `notification` object contains the following subfields:
  
  + `attributes`: List of attributes to be included in the notification message. If not specified (or empty), all attributes are included
    in the notification
  + `callback` : URL pointing to the service which will be invoked when a notification is generated. A NGSIv2
    compliant server must support `http` URL schema, other schemas (e.g. schemas for web sockets) could also be supported.
  + `throttling`: Minimal period of time in seconds which must elapse between two consecutive notifications. It
    is optional.
  + `timesSent` (not editable, only in GET operations): Number of notifications sent due to this subscription.
  
negative.code: * If there is more than one attribute exposing location which has been labeled as being the
  *default location*, then the query will be declared as ambigous and an HTTP error response with code ``409``
  must be sent. 
  
  ## Notifications formats
  
  Notifications include two fields: `subscriptionId` (which value is the one of the subscription triggering
  the notification) and `data` (which holds the actual data corresponding to entities and attributes).
  
  The `data` field can use different representation depending on two properties (especified as part of the
  corresponding subscription, see subscriptions operations section for details).
  
  * attrsFormat ("full", "keyValues", "values"; default is "full") to specify how the attributes are
    represented. See " Simplified Entity Representation" for details.
  * dataFormat ("array", "object"; default is "array"): to specify if the `data` field holding the information
    is an array or an object.
    * In "array" case, "data" is an array which elements are objects, each one representing an entity.
    * In "object" case, "data" in an object which keys are entity types and which values are object which keys, in sequence,
      are entity ids and which values is an object representing the entity.
  
  Combining both, there are 6 possibilities:
  
  * attrsFormat is "full" and "dataFormat" is "array":
  
  ```
  {
     "subscriptionId": "12345",
     "data": [
       {
         "id": "Room1",
         "type": "Room",
         "temperature": {
           "value": 23,
           "type": null,
           "metadata": {}
         },
         "humidity": {
           "value": 70,
           "type": "percentage",
           "metadata": {}
         }
       },
       {
       "id": "Room2",
       "type": "Room",
       "temperature": {
         "value": 24,
         "type": null,
         "metadata": {}
       }
     ]
  }
  ```
  
  * attrsFormat is "full" and "dataFormat" is "object":
  
  ```
  {
     "subscriptionId": "12345",
     "data": {
       "Room": {
         "Room1": {
           "temperature": {
             "value": 23,
             "type": null,
             "metadata": {}
           },
           "humidity": {
             "value": 70,
             "type": "percentage",
             "metadata": {}
           }
         },
         "Room2": {
           "temperature": {
             "value": 24,
             "type": null,
             "metadata": {}
           }
         }
       }
     }
  }
  
  ```
  
  * attrsFormat is "keyValues" and "dataFormat" is "array":
  
  ```
  {
     "subscriptionId": "12345",
     "data": [
       {
         "id": "Room1",
         "type": "Room",
         "temperature": 23,
         "humidity": 70
         }
       },
       {
       "id": "Room2",
       "type": "Room",
       "temperature": 24
       }
     ]
  }
  ```
  
  * attrsFormat is "keyValues" and "dataFormat" is "object":
  
  ```
  {
     "subscriptionId": "12345",
     "data": {
       "Room": {
         "Room1": {
           "temperature": 23,
           "humidity": 70
         },
         "Room2": {
           "temperature": 24
         }
       }
     }
  }
  
  ```
  
  * attrsFormat is "values" and "dataFormat" is "array":
  
  ```
  {
     "subscriptionId": "12345",
     "data": [ [23, 70], [24] ]
  }
  ```
  
  * attrsFormat is "values" and "dataFormat" is "object":
  
  ```
  {
     "subscriptionId": "12345",
     "data": {
       "Room": {
         "Room1": [23, 70],
         "Room2": [24]
       }
     }
  }
  
  ```
  
  ### Notification templates
  
  Based on the "template" field defined as part of the subscription and in a macro replacement language
  that could be used in "template" "callback", "headers" and "query". Details TBD.
  
  ## Group API Entry Point
  
  