Implemented GroupByMapValues

[maryum375]

Implemented a new function to GroupBy.
This Implementation allows to map the values on the same iteration.
Sending an identity function as the iterateeValue will de facto result in a normal GroupBy operation.

[samber]

i think we need to think a little bit about the naming

your variant: map then group by

other cool variant I use in my own projects: map then group by then aggregate values

[samber]

I would use a single iteratee: k, v := iteratee(item)