Return a global unique identifier for this agent instance.
        It's a combination of agent_id and instance_id:
          full_id = agent_id + '/' + instance_id

def get_full_id(self, state):
        """Return a global unique identifier for this agent instance.
        It's a combination of agent_id and instance_id:
          full_id = agent_id + '/' + instance_id
        """
        desc = state.medium.get_descriptor()
        return desc.doc_id + u"/" + unicode(desc.instance_id)