Fix input label for wildcard.
Add size to output msg.
Refactor task output naming into helpers.
Add size() to AtomicFile.
Adjust dir index parsing to new format.  Also start changing from partitions to labels.
Refactor task output into two classes, with BaseOutput format-agnostic.
Rename default shuffle stages.
Implement shuffle for group_node.
Fix use of label (instead of taskid) in reduce_input (test_simple now passes!).