code: def get_model_agents(self):
          
          model_stmts = self.get_statements()
          agents = []
          for stmt in model_stmts:
              for a in stmt.agent_list():
                  if a is not None:
                      agents.append(a)
          return agents
docs: Return a list of all Agents from all Statements.
  
          Returns
          -------
          agents : list[indra.statements.Agent]
             A list of Agents that are in the model.