Modify `non_backtracking_matrix` function's Return Type

[KeishiS]

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.28%. Comparing base (5878e7b) to head (d4206ae). Report is 26 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #328 +/- ## ========================================== - Coverage 97.30% 97.28% -0.03% ========================================== Files 115 118 +3 Lines 6796 6880 +84 ========================================== + Hits 6613 6693 +80 - Misses 183 187 +4 ```

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[gdalle]

Hi @KeishiS just checking in to see if you want to take care of the last changes or if I should?

[KeishiS]

I will commit the revised version within this week. Sorry for the delay.

[gdalle]

No worries, thanks!