Add adjacency tensor function

[maximelucas]

The function does not have a sparse option like the other matrix functions, because scipy.sparse has no implementation for sparse tensors (only 2d arrays). Potential candidate for later: the sparse library, but it does not have the equivalent of the csr_array that we usually use.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.53%. Comparing base (18de14b) to head (0614169).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #664      +/-   ##
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+ Coverage   93.50%   93.53%   +0.02%     
==========================================
  Files          64       64              
  Lines        5005     5028      +23     
==========================================
+ Hits         4680     4703      +23     
  Misses        325      325              

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[maximelucas]

@nwlandry I think I addressed your comments!

[nwlandry]

Looks great, @maximelucas! Thanks so much!