Some more useful helper / wrapper functions

[Jolanrensen]

Just a couple of small things I came across while working with the library.

Firstly, dataset.foreachPartition { } has overload resolution ambiguity, so I added forEachPartition {} to the API.

Secondly, some functions (like reduceGroups()) return a Dataset<Pair<K, V>>, or the user creates a key/value like dataset themselves and then it might be useful to just take the keys or the values (I know I found it useful).
So I added takeKeys() and takeValues() for Dataset<Pair>, Dataset<Tuple2>, and Dataset<Arity2>. It's a small thing, but might improve readability.

Lastly, just like getting the columns using property references, it might also be useful to sort datasets using those, so I added the ability to do:

val sorted: Dataset<YourClass> = unsorted.sort(YourClass::a)
val sorted2: Dataset<YourClass> = unsorted.sort(YourClass::a, YourClass::b)

@asm0dey let me know if these are helpful :)