Frames in the Market class

[Mv77]

@sbenthall and I were discussing the move towards a 'frame'-oriented architecture (#798) today. He mentioned that something he was thinking about was how to deal with aggregate variables like pLvlAggNow. The issue is that these variables do have transition equations and get updated in well-defined frames, but it is not clear what HARK object that frame should belong to.

I argued that frames updating aggregate variables should belong to some external entity that contains the agents, like the Market class that is currently implemented.

My argument is that I assume it is within HARK's vision to eventually allow users to create ABM-type models in which different AgentTypes can interact (say, construct an economy with both IndShockConsumerTypes and PortfolioConsumerTypes). In such an environment, it would be arbitrary to give either of the agent types the responsibility of updating aggregate variables like total factor productivity or pLvlAggNow. These transitions should, in my opinion, be executed by the object that contains the agents (a Market or Economy).

@sbenthall thought that the argument was persuasive and wanted me to share it with everyone else. Hopefully it can spur useful discussions.