Adding path function and tests

[jeremyguez]

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This PR is for adding the path function which outputs the path between nodes u and v. It is needed for the B2 index https://github.com/tskit-dev/tskit/pull/2327 (issue https://github.com/tskit-dev/tskit/pull/2252).

By the way, the path_length function (https://github.com/tskit-dev/tskit/issues/2249 https://github.com/tskit-dev/tskit/pull/2259) is maybe a little too constrained: if the tree has more than one root, the outputs is always inf. However, in non-rooted trees the paths between nodes that have the same root appear to be defined. For the moment, I defined the path function with this same constrain as in the path_length function, so both functions remain consistent.

cc @jeromekelleher

[codecov[bot]]

Codecov Report

Merging #2350 (91719f2) into main (a1b1e5d) will decrease coverage by 0.00%. The diff coverage is 92.00%.

@@            Coverage Diff             @@
##             main    #2350      +/-   ##
==========================================
- Coverage   93.27%   93.27%   -0.01%     
==========================================
  Files          28       28              
  Lines       26768    26793      +25     
  Branches     1217     1224       +7     
==========================================
+ Hits        24969    24992      +23     
- Misses       1766     1767       +1     
- Partials       33       34       +1     

Flag	Coverage Δ
c-tests	92.25% <ø> (ø)
lwt-tests	89.05% <ø> (ø)
python-c-tests	71.34% <4.00%> (-0.13%)	:arrow_down:
python-tests	98.82% <92.00%> (-0.03%)	:arrow_down:

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Impacted Files	Coverage Δ
python/tskit/trees.py	98.18% <92.00%> (-0.09%)	:arrow_down:

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[jeromekelleher]

By the way, the path_length function (#2249 #2259) is maybe a little too constrained: if the tree has more than one root, the outputs is always inf

I don't think this is true - if you ask for the length of a path that's within a given rooted subtree it'll work fine. We only check to see if the mrca is -1, and don't actually ask anything about multirooted trees in the implementation.

[jeremyguez]

By the way, the path_length function (#2249 #2259) is maybe a little too constrained: if the tree has more than one root, the outputs is always inf

I don't think this is true - if you ask for the length of a path that's within a given rooted subtree it'll work fine. We only check to see if the mrca is -1, and don't actually ask anything about multirooted trees in the implementation.

Oh yes right, sorry I got mixed up! Thanks for your answer @jeromekelleher.