Skip iteration graphs that put contraction before sparse output

[drhagen]

Fixes #53.

Ultimately, the issue was that merge_assignment was generating iteration graphs where sparse output layers were being iterated after contraction. This is not legal in the current design, but best_algorithm was picking one of them. Skipping these illegal graphs fixes the issue.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (bba845b) to head (18b6ddc).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   90.24%   90.29%   +0.05%     
==========================================
  Files          60       60              
  Lines        3126     3133       +7     
  Branches      368      370       +2     
==========================================
+ Hits         2821     2829       +8     
+ Misses        226      225       -1     
  Partials       79       79              

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