A modern programming language that makes developers happier.

kotlinlang

I realize kotlin-statistics has `xxxBy` methods, but it'd be handy to have extensions that'd work on the result of kotlin's `groupBy`. For example, it'd allow solving the "find sums by name length" example like so:
```
        val sumsByLengths = sequence
                .groupBy { it.name.length }
                .sumBy { it.value }
```

I never thought of this. Let me review this and compare it to the existing operators.

I can do a PR if it's something you'd accept