[ENH] add CITATION.cff file

[Moritz-Alexander-Kern]

A citation.cff file contains metadata about a software project. It can be used to generate citations for Elephant and link it to other sources of information, such as repositories, archives, or DOI services.

The citation.cff file can eventually replace the .zenodo.json file, which is currently used to provide metadata for Zenodo.

More information on the citation file format can be found here: https://citation-file-format.github.io/.

One of the current open issues with the citation.cff file is that it does not support specifying grants the project. This is a feature that is available in the .zenodo.json file and that is useful for acknowledging the financial support of the project. A related issue has been opened on the CFF GitHub repository: https://github.com/citation-file-format/citation-file-format/issues/491