Add reciprocity

[eriknw]

This gives us an example where we could use a cached property total_degrees that includes self-edges. Already in cluster.py, we need total_degrees excluding self-edges.

This is pretty short and sweet. How easy!

[codecov-commenter]

Codecov Report

Merging #6 (41246bd) into main (79f5598) will decrease coverage by 0.18%.
The diff coverage is 95.06%.

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   91.19%   91.01%   -0.19%     
==========================================
  Files           7        9       +2     
  Lines         409      434      +25     
  Branches       75       88      +13     
==========================================
+ Hits          373      395      +22     
- Misses         17       18       +1     
- Partials       19       21       +2     

Impacted Files	Coverage Δ
graphblas_algorithms/link_analysis.py	87.83% <80.00%> (ø)
graphblas_algorithms/reciprocity.py	91.42% <91.42%> (ø)
graphblas_algorithms/__init__.py	100.00% <100.00%> (ø)
graphblas_algorithms/cluster.py	87.07% <100.00%> (ø)
graphblas_algorithms/conftest.py	100.00% <100.00%> (ø)
graphblas_algorithms/tests/test_cluster.py	100.00% <100.00%> (ø)
graphblas_algorithms/tests/test_pagerank.py	100.00% <100.00%> (ø)
graphblas_algorithms/tests/test_reciprocity.py	100.00% <100.00%> (ø)

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