2. update datasets.

* rewrite the implementation of the marginalized kernel.
* implement four computing methods of the generalized random walk kernel.
* in the path kernel up to length h, use trie to save all paths, saving tremendous memories; use the Deep-first search to get paths from graphs.
* in model_selection_for_precomputed_kernel method, complete the part to do cross validation when Gram matrices are read from file.
* in get_dataset_attributes methods, correct three sub-methods about getting node degrees, add sub-methods to get fill factors of graphs.
* change default chunksize of pool.imap_unordered parallel method to 100.
* remove try... except blocks in case they hidden bugs.

MOD modify tree pattern kernel, remove tottering.
MOD save output in function model_selection_precpmputed.

* MOD the way to calculate WL subtree kernel, correct its results. - linlin
* ADD *kernel_train_test* and *split_train_test* to wrap training and testing process. - linlin
* MOD readme.md file, add detailed results of each kernel. - linlin

* MOD floydTransformation function, calculate shortest paths taking into consideration user-defined edge weight. - linlin
* MOD implementation of nodes and edges attributes genericity for all kernels. - linlin
* ADD detailed results file results.md. - linlin
* MOD Weisfeiler-Lehman subtree kernel and the test code. - linlin

* ADD delta kernel. - linlin
* MOD reconstruction the code of marginalized kernel. - linlin