Aggregator support in DataFrame

[Koert Kuipers]

i like the Aggregator a lot (org.apache.spark.sql.expressions.Aggregator),
but i find the way to use it somewhat confusing. I am supposed to simply
call aggregator.toColumn, but that doesn't allow me to specify which fields
it operates on in a DataFrame.

i would basically like to do something like
dataFrame
  .groupBy("k")
  .agg(
    myAggregator.on("v1", "v2").toColumn,
    myOtherAggregator.on("v3", "v4").toColumn
  )