set aggregate property on frame

[sbenthall]

This solves #1063 by setting the aggregate property of a variable on its frame in a FramedAgent.
It also pulls all the FramedAgent logic into its own modules frames

No new tests -- this is a refactor.
Some docs fixes

• Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• Updated documentation of features that add new functionality.
• Update CHANGELOG.md with major/minor changes.

[sbenthall]

@Mv77 could I ask you to review this? Should be quick.

[alanlujan91]

This looks good to me. I wonder if there are frames that could be joined, like PermShk and TranShk. Too many frames might become too complex to understand for new users. It would also be useful to map frames in the simulation with frames (or subperiods) in the solution method.

[alanlujan91]

where do these get defined now?

[alanlujan91]

nvm i found it

[sbenthall]

Yes, mapping simulation frames to the solution is a desired next step.

There seems to be some difference of opinion about the best way to group frames.
I prefer more granular frames. @mnwhite thinks more grouped frames might be better for performance reasons. I hear you saying you think more grouped frames are easier to understand.

The current implementation is agnostic about the best amount of aggregation in framing. That way performance of the same model, differently written, can be tested.

One benefit of granular frames is that they support the kind of plotting done in #1071