Add several [Tree assert methods](https://phytest-devs.github.io/phytest/reference.html#tree) and update docs.
- assert_branch_lengths
- assert_terminal_branch_lengths
- assert_internal_branch_lengths
- assert_no_negatives
- assert_tip_names
**Full Changelog**: https://github.com/phytest-devs/phytest/compare/v1.0.1...v1.1.0