Fix: initInstance hierarchy

  * This problem was detected after adding DomainBPlusTree.  Problem: there
    was a bug in the invocation of LeafNode.setEntries() from the constructor
    of LeadNode. When actually creating an instance of DomainLeafNode, it
    would invoke the override DomainLeafNode.setEntries(), which would attempt
    to access a yet uninitialized slot (domainEntries).  The solution is to
    change how initInstance happens by invoking it only once from the
    top-level AbstractDomainObject class, instead of invoking it once per
    class hierarchy level.