Name all iteration graph nodes as Node

[drhagen]

This makes the naming more consistent and clear.

[codecov-commenter]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (715ee8c) 88.66% compared to head (ebf7195) 88.66%.

Files	Patch %	Lines
src/tensora/desugar/to_iteration_graphs.py	53.12%	9 Missing and 6 partials ⚠️
src/tensora/iteration_graph/iteration_graph.py	83.33%	1 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   88.66%   88.66%           
=======================================
  Files          51       51           
  Lines        2938     2938           
  Branches      520      520           
=======================================
  Hits         2605     2605           
  Misses        255      255           
  Partials       78       78           

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