Param: all contents are static; contains the model parameters, so they can be
    easily shared and modified.

Position: represents an xy position on the range.  Contains a method that 
    returns the position that's one step to the north, south, east, west
    of this position.

Habitat: represents one habitat square on the grid.  Holds the current
    amount of grass on the habitat, and contains methods needed to update
    the amount of grass as time goes by (growth and eating).

Zebra: information about one of our zebras.  Contains methods to make our
    zebra do its tricks, including making choices, eating, and dying

Model: all information about the world we're simulating, including 
    appropriate collections of Zebra and Habitat objects.  The key methods
    are the constructor and a method to simulate one day in the model.

RunModel: class with the main() function that instantiates Model and runs
    everything.