Citation.cff not supported by zenodo

[FlorianK13]

In the latest release, the mirror to zenodo did not start due to a format issue in the citation.cff file.

[FlorianK13]

Resolved by using a .zenodo.json for now. However this is duplicated information in two files, I would remove this .json again and debug the CITATION.cff parsing of zenodo in the future.