Explicit feature extraction for the Weisfeiler-Lehman Subtree Kernel for Graphs.
The features of the kernel are trees resulting from breadth-first visits of
depth up to h.
The implementation is based on a relabeling process of the nodes of the
graphs (such novel labels are stored into an ad hoc field of the GraphNode object.
The only parameter of the kernel is the maximum depth of the visits, h.
In order to set the parameter, use the method setIterationNumber(int h);
 N. Shervashidze, “Weisfeiler-lehman graph kernels,” JMLR, vol. 12, pp. 2539–2561, 2011.