Add a citation file

[danielhuppmann]

Please add a citation.cff, see https://citation-file-format.github.io/cff-initializer-javascript

I suggest to use all (non-marginal) contributors as authors, and add the MESSAGEix manuscript (Huppmann et al., 2019), as a "related resource".

This is related to a request from the Horizon Europe Project PRISMA by @tscheypidi

fyi @khaeru @volker-krey

[glatterf42]

Where do we draw the line for contributors being 'marginal'? At least ten commits? This would exclude Gamze, but only for now. If she makes one more commit, we'd have to adapt this file. We could simply include everyone from https://github.com/iiasa/message_ix/graphs/contributors . I think it's only fair and doesn't hurt anyone.

[glatterf42]

Generally, I have not included any email addresses. I have tried to look people up on Orcid and added IDs whenever I found them. Same names still made trouble, though: with Miguel Poblete Cazenave, I have found Poblete as a surname, Cazenave as a surname, and Poblete-Cazenave as a surname, so currently, I set up Poblete as a second given name.
For the contributor lumbric, I couldn't find any name, so excluded them from the file.
Other notes I took in the process:

• Zenodo doesn't update itself automatically, so while e.g. I am a contributor now on Github, I'm not listed as such on Zenodo
• We might want to update version-specific stuff whenever that changes/becomes important
• With the doi as I have extracted it, one finds a 2018 version of the paper under some semi-obscure link. Which doi should be used?