Cluster model tests

* tests the Neighbors model. This model accepts an object with coordinates, and a collection of objects.  It returns the objects in the collection that are close to the first argument.
* tests the Common Neighbors model.  This model finds the group of items from collection that are neighbors of each member from another collection.
Yeah, not sure how to deal with this.  Instance variables I guess.  I thought methods were preferable, but they cause create a new object each time they're called. Maybe something with "|| =" ?