Provide public interface for `_convert_node_to_root_polytomy`

[mmore500]

This logic is only accessible via "private" i.e., _* access. Should it be made public? Probably.

Outside packages are depending on it (smirarab/sepp#125).

[mmore500]

Hi @simeonborko @smirarab --- would this API suit your use case?

https://github.com/jeetsukumaran/DendroPy/pull/194/files#diff-cd53d4c98adf0e3721dbe0f749ecf5dca68face2ed93a651633e079b27e4f43cR2376