CHANGE: Broadcast plugin

- Added file groupings by number of files: if not, then it's possible to create a Task with so many files that the TaskInputs table, InputVector field, gets too large and gets chopped at a random location (64k characters). Now assume default grouping of 1.
- Make the TargetSE always defined, even if the SourceSE list contains it: after all, why can't we copy the files to another SE of the same region.