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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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]

Thanks for the contribution! I made a few remarks but once these are addressed we can merge

[gdalle]

minor efficiency considerations, otherwise I think this will be good to go

[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!

[gdalle]

almost there, just a few things to tweak in the tests