Add topic tags

[Beliavsky]

I suggest adding the topics astrophysics, black-hole, black-holes, cuda-fortran, cuda in the About section, as explained at https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/classifying-your-repository-with-topics

[Daniel-Harrington]

Sure that sounds like a great idea, done. Thanks!