Add citation instructions and helper to prepare new releases

Add citation instructions in form of a CITATION.cff file, see https://citation-file-format.github.io/. The I filled the metadata in the cff file on the base of the entry already available in Zenodo (https://zenodo.org/record/3660963).

Since this is yet another place where the version number must be updated at every release, I added a small Python script to update all the relevant files in one go, including the code I use to generate a preliminary entry in CHANGELOG.md.