Fix adding mutation as parent operator

[YamLyubov]

• Mutation should be saved as a string to avoid problems with custom mutations serialisation
• Bandits should store arm action mapping accordingly: key should be a string

[pep8speaks]

Hello @YamLyubov! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers:

Comment last updated at 2024-06-05 10:17:55 UTC