Results 4 issues of SANDO Keishi

As mentioned in #326, I modified `non_backtracking_matrix` so that it returns a spase matrix.

enhancement

Thank you for your ongoing development efforts. I have a question concerning the `non_backtracking_matrix` function. I noticed it returns a non-sparse matrix, whereas the `adjacency_matrix` function returns a sparse matrix....

Thank you for sharing graph kernels implementation. I fixed 2 errors caused by the update of numpy. * Some older numpy data types are used in `GKextCPy_wrap.cxx`. However, such older...

Thank you for sharing your implementation. While running the provided code, I encountered a few issues that required modifications. I believe these changes could also be helpful for others, so...