The experiment can now transfer data collections

The transfer tracker publishes the "globus.end transfer" composition transition for
each data collection in a transfer.

App now transfers several data collections with random files at a time.

NOT TESTED yet
Added paging in GlobusTransferTracker's inner loop

(Publishing the composition transition between globus and analysis.)
Initial version of the dataset extractor

This Python script will simulate the extraction of metadata from
successful Globus transfers and trigger the composition transition
with the "metadata" life cycle for each such data set.