Define branch length over root to be zero.

[jeromekelleher]

Closes #188.

[codecov]

Codecov Report

Merging #191 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #191      +/-   ##
==========================================
+ Coverage   85.95%   85.95%   +<.01%     
==========================================
  Files          16       16              
  Lines        8728     8732       +4     
  Branches     1677     1678       +1     
==========================================
+ Hits         7502     7506       +4     
  Misses        733      733              
  Partials      493      493

Flag	Coverage Δ
#c_tests	85.95% <100%> (ø)	⬆️
#python_tests	98.27% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
python/tskit/trees.py	98.61% <100%> (ø)	⬆️

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[jeromekelleher]

Any objections to this @hyanwong? Seems like a straightforward improvement that's unlikely to break people's code (I doubt anyone depends on the current exception-throwing behaviour).

[hyanwong]

I have no objections.