Implement, test polytomize_root

[mmore500]

As a public interface for _convert_node_to_root_polytomy

Closes #169

[codecov-commenter]

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All modified and coverable lines are covered by tests ✅

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@@           Coverage Diff           @@
##             main     #194   +/-   ##
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  Coverage        ?   61.63%           
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  Files           ?      105           
  Lines           ?    18651           
  Branches        ?        0           
=======================================
  Hits            ?    11495           
  Misses          ?     7156           
  Partials        ?        0           

Flag	Coverage Δ
unittests	61.63% <100.00%> (?)

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