Support flatMap function in GroupedStream

[ni-ze]

flatmap function takes one element and produces zero, one, or more elements. such as, a flatmap function splits sentences to words:

flatmap("hello world") -> "hello" "world"

[selectbook]

please assign it to me

[joeCarf]

I'd like to do this. plz assign it to me

[elenacliu]

Can you assign it to me? I‘m working on this these days.