Do a full iteration graph merge when addition terms have no contractions

[drhagen]

Stop trying to distribute multiplication over a SumNode when generating iteration graphs.

Fixes #50.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 74.28571% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.48%. Comparing base (b9d9c49) to head (b676697).

Files with missing lines	Patch %	Lines
src/tensora/desugar/_to_iteration_graphs.py	74.28%	2 Missing and 7 partials ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   90.29%   90.48%   +0.18%     
==========================================
  Files          60       60              
  Lines        3133     3162      +29     
  Branches      370      384      +14     
==========================================
+ Hits         2829     2861      +32     
+ Misses        225      219       -6     
- Partials       79       82       +3     

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