Fix bug in CMA-ES introduced in #240

[btjanaka]

Description

In #240, we introduced a slight change to CMA-ES that resulted in buggy code -- in particular, we received warnings about nan values in sqrt in the tests. This PR reverts that bug.

TODO

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