Single Line Change.  Cluster::nodeList type change from List<Node> to 
ArrayList<Node>.
Serialization of Domain classes, continued refactoring of class Cluster
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Made Domain classes Cluster, Node, and Disk implement Serializable and
added private static final long serialVersionUID to each.

In class Cluster added public void addNodesToCluster() and removed
several methods that will not be used by the class or application