Use CITATION.cff standard for citation information

[JaroCamphuijsen]

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Change from CITATION.md to CITATION.cff and use the proper data standard for .cff files. This will also enable the CITATION.cff-github integration and show the citation button in the web UI.

Additionally removed the AUTHORS file in the root of the repository. It is only mentioned (and linked to wrongly) in the paper.md file and contains duplicate information that already diverged from the content of the original CITATION.md. I added the additional authors from the AUTHORS file in the new CITATION.cff file.

Currently, the authors in CITATION.cff only have a given name and family name. It would be good to add an ORCID for everyone, perhaps this is something to ask in the slack channel/mailinglist and/or through other channels? I'm happy to put them all in if the authors post their ORCID as a comment to this PR.

fixes #477