Implement JSTOR-compliant ONIX 3.0 XML output

[rhigman]

Fixes #44 by creating a new, separate ONIX output, based on the Project MUSE output, which fixes the minor issues noted in #44 (comment).

We are still awaiting feedback from a) JSTOR, to confirm that the changed format does now meet their requirements, and b) Project MUSE, to comment on whether the changed format would be acceptable to them.

If Project MUSE have no issues with the minor changes, we should ideally merge the separate MUSE and JSTOR outputs back into a single "flavour". This PR allows users to create JSTOR-compliant output in the meantime.

[ja573]

I guess that even if MUSE accepts this specification we should keep them separate, but link them under platform, e.g. onix_3.0::jstor -- accepted_by--->["/platforms/jstor", "/platforms/project_muse"]

[rhigman]

I guess that even if MUSE accepts this specification we should keep them separate, but link them under platform, e.g. onix_3.0::jstor -- accepted_by--->["/platforms/jstor", "/platforms/project_muse"]

Good point. Since MUSE said they could only accept titles with at least one BIC/BISAC and didn't want Keywords included, I'm about to make that change to the MUSE specification, which would make it diverge from the JSTOR specification anyway.

[rhigman]

JSTOR have now confirmed that this format meets their specifications.

As mentioned above, MUSE have stricter requirements for subject codes, so would not be able to ingest this format. However, they didn't mention any objections to the changes JSTOR initially requested (listed under #44 (comment)).