Add CITATION.cff files to all repositories

[samdupton]

GitHub supports the inclusion of CITATION.cff files which give citation information for software. This information appears in the sidebar of the repository under "Cite this repository", e.g. here. It also ensures that the project has the correct metadata when importing into Zenodo.

It might be worth adding these files to all of the BHPToolkit repositories. They can be generated using the cffinit website.

[barrywardell]

This would be really useful, particularly as it would streamline the process of linking with Zenodo. Right now, whenever a release of a package is created it is necessary to manually edit the Zenodo page afterwards. Hopefully this would remove that requirement.

[nielsw2]

Coincidentally, a pull request was just created to add such a file to FEW: BlackHolePerturbationToolkit/FastEMRIWaveforms#112. I agree that we should add them to all the packages.

[barrywardell]

I have added files in the Teukolsky and QNM-Mathematica packages.