add pbs function to tree.py

[atongsa]

Description

add a PBS function to test population local adaptation;
solve the issue #2717 ;

Fixes #(2717) <- Putting the issue number here will auto-close the issue when this PR is merged

PR Checklist:

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

[benjeffery]

Thanks @atongsa!
It looks like your PR has a couple of extra files (the ones in tmp) can you remove those?

Also do you want some pointers on adding a test for this new function, or are you happy to try? There are some pointers at https://tskit.dev/tskit/docs/stable/development.html#tests

[atongsa]

Thanks @atongsa! It looks like your PR has a couple of extra files (the ones in tmp) can you remove those?

Also do you want some pointers on adding a test for this new function, or are you happy to try? There are some pointers at https://tskit.dev/tskit/docs/stable/development.html#tests

i will try add a test