Python model configuration

[sbenthall]

Based on the meetings today: the simulation and solution aspects of HARK will need to both refer to a single, canonical description of a model.

This model configuration should define:

• the model variables (including states, controls, and shocks)
• initial values for the variables
• transition equations for the state variables
• exogenous distributions or processes for the shocks
• anything needed by the solvers for the controls
• constraints (Gamma)
• parameters

A major design question that has not been resolved yet is how to bundle this model description up into frames/stages. See #798

Another question raised by this is how models should inherit configurations from each other. Currently, model inheritance is done through Python's class inheritance. But as more of a model's specification is removed from HARK class code and into a configuration object, it opens up other options. In any case, the class inheritance is not quite standardized yet across the library and could use some thought.

[sbenthall]

Will need to revisit this towards the end of the push to 1.0 to see what smaller parts can be accomplished

[sbenthall]

will split this into separate issues and close.

[mnwhite]

Closing and archiving as for HARK 1 roadmap. See Seb's tags for where it's being actively worked on.