feat: array unique flatmap for flattening array returns

[JacobBanghart]

Description

Use flat map in ArrayUnique to allow for the returning of nested arrays.
Other: I am a terrible wordsmith and would love some thought about how to document this well.

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• tests are added for the changes I made (if any source code was modified)
• documentation added or updated
• I have run the project locally and verified that there are no errors

Fixes

fixes #2137,