Added creation of a subset node that requires both a parent_node and parent_index field be defined from which the subset node will be created with a subset of the data contents based on a third required parameter subset_indices. The first two parameters are required at node creation time, while the third can be added with a PUT at a later time.  Still need to make the node data immutable after subset_indices are uploaded and add/test fuctionality for several features of subset node operation.