Fix dropped edges during a coalesce operation

[kuanb]

Fixes #57

Conveniently leverages the new simplify operation added to resolve lost edges and hanging nodes issue.

Example of LA (which was not working before) working now:

[codecov]

Codecov Report

Merging #64 into master will increase coverage by 0.17%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   90.28%   90.45%   +0.17%     
==========================================
  Files          12       12              
  Lines         741      744       +3     
==========================================
+ Hits          669      673       +4     
+ Misses         72       71       -1

Impacted Files	Coverage Δ
peartree/toolkit.py	96.4% <80%> (+0.67%)	⬆️

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