Migrate codemeta.json to citation.cff

Currently the project use a codemeta.json and planetary-coverage.bib file to reference and format the citation of the project.

We recommend to migrate these files to a single citation.cff file, that is considered a standard for citation file. It is natively supported by Software Heritage, Zotero, Zenodo and Github (but not yet by Gitlab).

Export to codemeta.json and bibtex is also possible with cffconvert. These files can be auto-generated and stored in a citation/ folder.