Change Tree.unrank order of arguments to feel more natural

[daniel-goldstein]

Description

Changes the order of the arguments to Tree.unrank and combinatorics.num_labellings to take first the number of leaves and then the rank of the tree in question.

Fixes #950

cc: @hyanwong @jeromekelleher

PR Checklist:

• Tests that fully cover new/changed functionality.
• Documentation including tutorial content if appropriate.
• Changelogs, if there are API changes.

[AdminBot-tskit]

📖 Docs for this PR can be previewed here

[codecov]

Codecov Report

Merging #978 (845693c) into main (b47cd2c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #978   +/-   ##
=======================================
  Coverage   93.40%   93.40%           
=======================================
  Files          26       26           
  Lines       20395    20395           
  Branches      825      825           
=======================================
  Hits        19050    19050           
  Misses       1308     1308           
  Partials       37       37           

Flag	Coverage Δ
c-tests	92.43% <ø> (ø)
lwt-tests	93.66% <ø> (ø)
python-c-tests	94.38% <100.00%> (ø)
python-tests	98.53% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
python/tskit/combinatorics.py	99.13% <100.00%> (ø)
python/tskit/trees.py	97.25% <100.00%> (ø)

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[benjeffery]

LGTM, thanks a lot @daniel-goldstein!

[benjeffery]

Gah, I forgot to squash. Sorry, only 3 so no biggie.