dev(hansbug): add setdefault method to TreeValue

[HansBug]

#44

[codecov]

Codecov Report

Merging #58 (da1e6e2) into main (951e7b7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #58   +/-   ##
=======================================
  Coverage   99.25%   99.26%           
=======================================
  Files          35       35           
  Lines        2155     2165   +10     
=======================================
+ Hits         2139     2149   +10     
  Misses         16       16           

Flag	Coverage Δ
unittests	99.26% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
treevalue/tree/common/storage.pyx	100.00% <100.00%> (ø)
treevalue/tree/tree/tree.pyx	100.00% <100.00%> (ø)

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