check MARC relators for author/narrator role support

[daryanypl]

conversation up to now:
[Darya] Metadata attribute needs to handle multiple authors and narrators. May be nice to be able to specify contribution, for example: “narrated by Bob, with introduction narrated by Alice”.

[Hadrien] They already do, just use an array: "creator": ["Author 1", "Author 2"].
I don't think that there are roles specific enough to express "intro narrated by Alice" though, but I'll have to look at MARC relators to be fully sure.

[Darya] Importance of this functionality depends on library size and popularity. If we postulate a large collection and an active userbase, users can feel disappointed, if they see a famous name on the contributor list, only to find out that they did a quickie intro. For current state, we’re OK. Although, if we say that the audiobook manifest is to be used is to be used to also generate the opds feed entry, then the extra information of author (as well as narrator) roles becomes important, for sorting.

[HadrienGardeur]

MARC relators are available at: https://www.loc.gov/marc/relators/relaterm.html

There's an "author for the introduction" (aui) but I'm not aware of any role for "introduction narrator".

[HadrienGardeur]

I've been thinking about this some more and here are a few thoughts:

• currently, we favour existing contribution roles expressed in schema.org over a generic contributor element with a MARC relator (and I believe that this is the right decision)
• but when a contribution can't be expressed using schema.org, we fallback to generic role + MARC relator
• whereas with subject codes, we allow multiple schemes

We might want to consider the same extension mechanism for the generic contributor element, and allow other schemes.

This means that a contribution using MARC would be expressed as:

"contributor": {
  "name": "John Doe",
  "scheme": "https://www.loc.gov/marc/relators/",
  "role": ["mrk"]
}

[daryanypl]

After offline conversation, decided to keep "author", "narrator" as distinct elements, if need to add "illustrator" or such, then can use contributor element. See if future usage warrants making the spec more complicated, and how much we might need primary authors explicitly defined (as opposed to using the position of the name element in document).

[HadrienGardeur]

We already have a much longer list defined: https://github.com/readium/webpub-manifest/blob/master/contexts/default/definitions.md#contributors