Add citation metadata in machine-parsable formats

[alvorithm]

This adds two popular citation metadata formats, based on the templates offered by the Astrophysics Source Code Library, see recent entry added for sbi at https://ascl.net/2306.002 (https://ascl.net/2306.002/CITATION.cff, https://ascl.net/2306.002/codemeta.json).

• https://citation-file-format.github.io/ (used e.g. by the Zotero plugin when bookmarking).
• https://codemeta.github.io/ (with a long list of crosswalks that help other projects map metadata to their own packaging files, e.g. debian packages, wikidata, etc).

These citation files pertain to the software, but they encourage to cite the paper. There are best practices about citing the software directly that result in nuances in these files (e.g. use of 'preferred citation') that I have not investigated.

The paper citations are encouraged to occur not via the ASCL or the JOSS URLs but as advised by ASCL engineer Alice Allen through the DOI resolver URL.

Finally, note that this PR does not deal with CI updates to this metadata (notably concerning version numbers and release dates). See

• https://github.com/caltechlibrary/codemeta2cff to get a GH action to autoupdate the CFF file from the codemeta.json file.
• https://codemeta.github.io/tools/ for more pointers (e.g. automatic generation)

[codecov]

Codecov Report

Merging #852 (51d8b41) into main (4e29490) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #852   +/-   ##
=======================================
  Coverage   74.60%   74.60%           
=======================================
  Files          80       80           
  Lines        6238     6238           
=======================================
  Hits         4654     4654           
  Misses       1584     1584           

Flag	Coverage Δ
unittests	74.60% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[janfb]

that's great, thanks for adding it!