Support for SIP export of arbitrary portions of the Arrangement

[gregjan]

Sometimes you want to tease apart an incoming file set into multiple collections or portions thereof. You could do this with top level folders, but it would be more useful if you could export such structures separately.

For instance, if you defined two collections within the arrangement and sorted materials into them. You'd want to export those collections separately. They might be destined for separate repositories or storage systems and they might belong in separate archival packages.