replace HARKobject with MetricObject and Model

[sbenthall]

This PR implements a change brought up in issue #612

It replaces HARKobject with two classes, MetricObject, which has the distance metric functionality, and ParameterizedObject, which has the parameter assignment functionality.

The motivations for this are:

• there is no use case for using the distance functionality on agents; this is only used for elements of a solution
• the uses of assignParameters in solution elements could be written in the same number of lines using standard Python constructor formatting
• there are many reasons why parameter assignment for models may need to be handled in a special way (see Equivalency between 2 models/agents.  #612 separate out model definition, solution method parameters, and simulation parameters #660) that is not assigning them to object attributes (which is normally what constructors do). So preserving the ParameterizedObject superclass will make it easier to maker these other changes.

• 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]

Latest commits here continue on this trajectory of work:

• renamed ParameterizedObject class to Model; this now corresponds to the class with the same name in Dolo: https://github.com/EconForge/dolo.py/blob/master/dolo/compiler/model.py#L523
• Adding a __repr__ to Model so that parameters are printed upon the inspection of the object. Similar to Dolo: https://github.com/EconForge/dolo.py/blob/master/dolo/compiler/model.py#L641-L698 This addresses Show model equations for inspected HARK AgentType and Market classes #479 as much as possible without Frames.

[sbenthall]

Now, Model class has equivalency method __eq__, fixing #612
Asking @MridulS to review

[sbenthall]

The following tickets are being resolved or blocked by this PR:

• Show model equations for inspected HARK AgentType and Market classes #479
• separate out model definition, solution method parameters, and simulation parameters #660
• Model parameter getters and setters modeled on NetworkX methods #679
• Equivalency between 2 models/agents.  #612
• the "whiteboard problem" #889