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.
I would like to make this DRYer.  But I can't just reference a method, because I need the objects in "include" to be identical to the objects in the collection. If I use a method it just creates new objects with the same properties.  Recommendations appreciated!