'to_nexus()' emits basically compliant Nexus

[jeetsukumaran]

Description

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PR Checklist:

• Tests that fully cover new/changed functionality.
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[codecov]

Codecov Report

Merging #1835 (c92329c) into main (6b4d127) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1835   +/-   ##
=======================================
  Coverage   93.43%   93.43%           
=======================================
  Files          27       27           
  Lines       24803    24804    +1     
  Branches     1094     1094           
=======================================
+ Hits        23174    23175    +1     
  Misses       1594     1594           
  Partials       35       35           

Flag	Coverage Δ
c-tests	92.20% <ø> (ø)
lwt-tests	89.14% <ø> (ø)
python-c-tests	94.53% <100.00%> (+<0.01%)	⬆️
python-tests	98.75% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
python/tskit/trees.py	97.84% <100.00%> (+<0.01%)	⬆️

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[jeromekelleher]

Great, thanks @jeetsukumaran! I'll follow up with a PR to add dendropy based testing.