@pchlupacek I added this line because `def chunk[I](n: Int): Process1[I,Vector[I]]` also throws an exception for non-positive chunk sizes and I thought both functions should be consistent when it comes to handling nonsensical input. I don't have a strong opinion whether the exception should be thrown immediately or wrapped into `Halt`. Maybe @pchiusano can comment on this.

And btw, without this line, `window` also throws an exception for `n <= 0` because `tail` is called on an empty `Vector`.