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.
@elijahkim it's not that weird. I'm more confused by what's going on. You have a method that is plural, yet it's returning only one thing... strange