[documentation] Doc strings and light code gardening

[kuanb]

In addition to doc strings:

• move where SyntheticTransitNetwork validation occurs to path level operation
• break out average wait calculation from synthesized headways to a lambda operation (held in unique_edge_operator)

[codecov[bot]]

Codecov Report

Merging #116 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
+ Coverage   91.72%   91.73%   +<.01%     
==========================================
  Files          13       13              
  Lines        1051     1052       +1     
==========================================
+ Hits          964      965       +1     
  Misses         87       87

Impacted Files	Coverage Δ
peartree/graph.py	97.11% <100%> (-0.03%)	:arrow_down:
peartree/graph_tool.py	88.6% <100%> (ø)	:arrow_up:
peartree/summarizer.py	97.5% <100%> (ø)	:arrow_up:
peartree/paths.py	96.61% <100%> (+0.11%)	:arrow_up:

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