The `Collection` entry is part of the catalog, not of the document, so
its logic should be placed there instead. The document should look in the
catalog to fetch it, and not have knowledge of `catDict`, which is a
member internal to the catalog.
Moreover, remove the collection member from the document instance. It's
only used internally and was also never intended to be public. For users
it's exposed by the `getMetadata` API endpoint as `IsCollectionPresent`.
Moving this out of the `parse` function makes sure that the getter is
only executed if the document information is actually requested
(potentially making initial parsing a tiny bit faster).
b41a2f4d5a