Adding B2 index and tests

[jeremyguez]

Here is the new PR for the B2 index.
@jeromekelleher

[codecov]

Codecov Report

Merging #2353 (9279607) into main (22d4a50) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2353   +/-   ##
=======================================
  Coverage   93.29%   93.29%           
=======================================
  Files          28       28           
  Lines       26768    26781   +13     
  Branches     1228     1232    +4     
=======================================
+ Hits        24973    24986   +13     
  Misses       1762     1762           
  Partials       33       33           

Flag	Coverage Δ
c-tests	92.24% <ø> (ø)
lwt-tests	89.05% <ø> (ø)
python-c-tests	71.15% <7.69%> (-0.06%)	⬇️
python-tests	98.86% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
python/tskit/trees.py	98.34% <100.00%> (+0.01%)	⬆️

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[benjeffery]

It's no biggie so no worries but as a quick tip - you can force-push anything to an existing PR, so there's usually no need to make a new one.

git checkout -b fix_pr_branch
.... fix things ....
git checkout original_broken_branch
git reset --hard fix_pr_branch
git push -f repo_name original_broken_branch

[jeremyguez]

Thank you @benjeffery for the tip, very useful!