Use csr_matrix() constructor in hash_graph_kernel.py

[jomora]

Hey,
executing wl.weisfeiler_lehman_subtree_kernel with use_gram_matrix = True and kernel_parameters_wl = [3, True, False, 0] gives the following error.

Traceback (most recent call last):
  File "hash_graph_kernels.py", line 156, in <module>
    main()
  File "hash_graph_kernels.py", line 130, in main
    scale_attributes=True, lsh_bin_width=1.0, sigma=1.0, use_gram_matrices=True)
  File "/media/sf_SEML/code/examples/hashgraphkernel/graphkernel/hash_graph_kernel.py", line 52, in hash_graph_kernel
    feature_vectors = feature_vectors.tocsr()
AttributeError: 'numpy.ndarray' object has no attribute 'tocsr'

By using feature_vectors = sparse.csr_matrix(feature_vectors) this works out.
@chrsmrrs , what do you think?