Add FlatMap Operation

[AlexandruGG]

This PR adds the FlatMap Operation.

• has tests
• has static analysis check
• has documentation
• clarify key types and normalize usage

Bonus, static analysis checks:

• flatten
• unwrap

[drupol]

I really like the fact that the map callback can return any type of iterable, this will still continue to work !

[AlexandruGG]

I really like the fact that the map callback can return any type of iterable, this will still continue to work !

Nice observation! I'll add some unit tests to the relevant operations to ensure that behaviour doesn't break by accident

[AlexandruGG]

@drupol I think this is ready but have a look at the types, tests, and docs and see if everything is as you'd expect from this

[drupol]

All good, maybe next time it would be better to separate a new feature from the small refactoring, I prefer when PRs are clearly scoped.