Genetic Algorithm: Fix bug in multi-threading (#12644)

* Fix bug in multi-threading

- Multi-threading (despite being commented out) had a tiny bug: missing target argument (2nd argument).
- Commented out code was also slightly hard to understand, added (Option 1/2) in comments to clarify where a user may choose between 2 implementations.

* Update basic_string.py

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Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>