the "whiteboard problem"

[sbenthall]

Throughout HARK, the AgentType object is being used like a whiteboard.

Mathematical objects are assigned as attributes to self willy-nilly. This leads to an unstructured namespace that is hard to understand.

As a further refactoring step, aligned with #660 and other issues:

• compartmentalizing the math so that more of the board is "erased" between operations
• when possible, the math "inputs" into functions get passed in as arguments rather than getting drawn from self.

This will encapsulate the code in a way that makes it easier to decompose and refactor.