Separate true model parameters from the parameters used for approximation

[sbenthall]

A true model is an infinite, continuous model with a single solution.

There are many ways to approximate such a model with finite agents and discretization of continuous spaces.

The parameters pertaining to approximation shall be separate from the parameters of the true model.

[sbenthall]

One approach to this has been implemented already by @alanlujan91 in the SHARKFin AgentPopulation class.

[mnwhite]

Archiving this as part of the HARK 1 roadmap.