Fix `pivot` functions API

[AndreiKingsley]

Now most of pviot, pivotMatches, pivotCounts overloads use common ColumnsSelector instead of PivotDsl columns selector.

[Jolanrensen]

Yes, I would even consider this a bug!

Stupid I can write

df.pivot { city then age }.groupByOther().count()

but not

df.pivotCounts { city then age }