Do not require an immutable.IndexedSeq for process1.repartition

This PR changes `process1.repartition` to require a split function of type `I => scala.collection.IndexedSeq[I]` instead of `I => scala.collection.immutable.IndexedSeq[I]`. This allows to use for example `(_: String).split("\n")` (which has type `String => Array[String]`) as parameter for `repartition` without the need to convert the `Array` into an `immutable.IndexedSeq`