Doc cleanup
Cleaned up methods on DocumentaryUnit and Authority (they won't be
linked directly but via annotations.) Also renamed ModelTest to
reflect the fact it's actually testing the fixture data.
Move the DatePeriod model to be a relation of the DocumentDescription,
rather than the DocumentaryUnit, since it's "historical" data and may
well vary between descriptions. This complicates some tests, but we're
now using the scary magic in the BundleUtils class to take care of that.
Use a read-only database for certain commands