Error on unary nodes in KC distance

[daniel-goldstein]

The KC distance is only defined on full trees and unary nodes greatly affect distances. kc_distance now errors on trees with unary nodes. Users should run simplify on trees before comparing.

Resolves #487

cc @jeromekelleher @hyanwong

[codecov]

Codecov Report

Merging #508 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #508   +/-   ##
=======================================
  Coverage   86.81%   86.82%           
=======================================
  Files          21       21           
  Lines       15969    15979   +10     
  Branches     3102     3105    +3     
=======================================
+ Hits        13864    13874   +10     
  Misses       1055     1055           
  Partials     1050     1050           

Flag	Coverage Δ
#c_tests	87.74% <100.00%> (+0.01%)	⬆️
#python_c_tests	90.52% <ø> (ø)
#python_tests	99.18% <ø> (ø)

Impacted Files	Coverage Δ
c/tskit/core.c	90.47% <100.00%> (+0.06%)	⬆️
c/tskit/trees.c	90.74% <100.00%> (+0.02%)	⬆️

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[jeromekelleher]

LGTM, thanks @daniel-goldstein!