Make it obvious how to build a collection from others' definitions

[rybesh]

... (optionally?) forking some (all?) of those definitions.

Adam put it well: "you should be able to create the Roman Amphitheater collection (authority: Sebastian Heath), for which you could adopt periods from Boatwright (for the moment adding a note to indicate which are derivedFrom Boatwright) and then make up the ones you're missing, assigning them the Roman Empire as spatial coverage."