Negative edge cost fallback, tag mode

[kuanb]

Two items in this PR:

• We take the absolute value of all edges as there are currently negative edges being calculated
• Note this is a fallback since these are "edge cases" in that they have to do with schedule data being staggered incorrectly, so we are just "wrapping around" the start/end of the schedule
• Adding a tag for walk segments when they are added to make it easy to identify walk transfers when they are generated

[codecov[bot]]

Codecov Report

Merging #40 into master will decrease coverage by 0.06%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   92.19%   92.12%   -0.07%     
==========================================
  Files          10       10              
  Lines         538      546       +8     
==========================================
+ Hits          496      503       +7     
- Misses         42       43       +1

Impacted Files	Coverage Δ
peartree/graph.py	96.49% <100%> (ø)	:arrow_up:
peartree/summarizer.py	96.72% <100%> (+0.01%)	:arrow_up:
peartree/plot.py	92.85% <0%> (-7.15%)	:arrow_down:

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[kuanb]

Before merging, add test assertions