NPMF for ConsIndShock

[sbenthall]

The New Python Model Format (see #1371) has been used so far for a very simple model: infinite horizon perfect foresight.

A clear next step is to implement a similar file for ConsIndShock and at it to the model library: https://github.com/econ-ark/HARK/tree/master/HARK/models

In principle, this would enable to ConsIndShock model file to be refactored to use the generic Monte Carlo simulator, rather than its own custom simulation code.

Since there are so many variations of the ConsIndShock model supported by the current object-oriented version of the model, this task also involves putting some thought into how to efficiently encode multiple models which are slight variations of each other. This work will be roughly analogous to the way that YAML model configurations can be built off of each other.