Do not require an immutable.IndexedSeq

An immutable IndexedSeq for the split function of process1.repartition is a too
specific requirement. Allowing also mutable.IndexedSeq has the advantage that
(s: String) => s.split(x) (i.e functions that split into arrays) are valid
parameters.
Merge pull request #113 from fthomas/repartition

Do not require an immutable.IndexedSeq for process1.repartition