Question regarding non_backtracking_matrix function's return type

[KeishiS]

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. Can I ask you for some insight into the reasons behind this choice?

If there's no specific reason for using a non-sparse matrix, I'd love to submit a pull request to modify the function so that it returns a sparse matrix.

Thanks for your time!

[gdalle]

I don't think there is a reason for it not to be sparse, feel free to submit a PR fixing it! Review may take a while but if the PR is simple enough we shouldn't have any issues