Include threshold in `archive.best_elite`

[btjanaka]

Description

Previously, we popped the threshold from best_elite to maintain backwards compatibility. This PR adds the threshold back so that best_elite aligns with the other archive methods.

TODO

• [x] Make tighter tests for best_elite
• [x] Add threshold back to best_elite (i.e., no longer pop it)

Questions

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go