Bugfix: sample pair from sampled triplet, not all leaves

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.94%. Comparing base (f361e4d) to head (c97b934).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #142 +/- ## =========================================== - Coverage 97.71% 75.94% -21.78% =========================================== Files 632 632 Lines 8720 8720 =========================================== - Hits 8521 6622 -1899 - Misses 199 2098 +1899 ``` | [Flag](https://app.codecov.io/gh/mmore500/hstrat/pull/142/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Andres+Moreno) | Coverage Δ | | |---|---|---| | [tests](https://app.codecov.io/gh/mmore500/hstrat/pull/142/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Andres+Moreno) | `75.94% <ø> (-21.78%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthew+Andres+Moreno#carryforward-flags-in-the-pull-request-comment) to find out more.

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