Refactor implementation of bidirectional synthetic network

[kuanb]

• Reuses nodes created from first direction
• Generated both edge directions in the same loop/add edge step
• Updates tests

[codecov[bot]]

Codecov Report

Merging #34 into master will decrease coverage by 0.2%. The diff coverage is 94.73%.

@@            Coverage Diff            @@
##           master     #34      +/-   ##
=========================================
- Coverage   89.61%   89.4%   -0.21%     
=========================================
  Files          10      10              
  Lines         539     538       -1     
=========================================
- Hits          483     481       -2     
- Misses         56      57       +1

Impacted Files	Coverage Δ
peartree/graph.py	96.49% <94.73%> (-0.91%)	:arrow_down:

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