Implement is_root

[hyanwong]

Description

Fixes #2620

PR Checklist:

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

[codecov]

Codecov Report

Merging #2669 (f70a1a6) into main (1167966) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2669   +/-   ##
=======================================
  Coverage   93.94%   93.94%           
=======================================
  Files          28       28           
  Lines       27812    27814    +2     
  Branches     1281     1281           
=======================================
+ Hits        26127    26129    +2     
  Misses       1648     1648           
  Partials       37       37           

Flag	Coverage Δ
c-tests	92.26% <ø> (ø)
lwt-tests	89.05% <ø> (ø)
python-c-tests	71.94% <50.00%> (-0.01%)	⬇️
python-tests	98.98% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
python/tskit/trees.py	98.75% <100.00%> (+<0.01%)	⬆️

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[benjeffery]

Thanks @hyanwong! I've made some suggestions about testing.
I'm confused about how Tree.roots takes account of root_threshold, but I assume I'm missing something.

[jeromekelleher]

LGTM