Custom implementation of simplify_graph

[kuanb]

Very much akin to the method of the same name in OSMnx (and have made sure to cite the function in the comments of the method). Has some updates that help with using the method with peartree, names:

• no list assembly of other misc. attributes
• checks to ensure that we don't simplify walk and transit mode segments

Note: Also relevant as a "good enough" replacement for the desired feature add in Issue https://github.com/kuanb/peartree/issues/46

Example, San Mateo: Before:

After:

[codecov[bot]]

Codecov Report

Merging #61 into master will decrease coverage by 3.27%. The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   91.66%   88.39%   -3.28%     
==========================================
  Files          11       12       +1     
  Lines         672      741      +69     
==========================================
+ Hits          616      655      +39     
- Misses         56       86      +30

Impacted Files	Coverage Δ
peartree/toolkit.py	95.73% <95.23%> (-0.21%)	:arrow_down:
peartree/convert.py	0% <0%> (ø)

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