CITATION.cff fix and automatic validation of your citation metadata

[abelsiqueira]

Hello!

We noticed that your CITATION.cff had a small issue and fixed it - possibly by just updating cffversion to 1.2.0. In addition to the fix, this Pull Request automates validation of the CITATION.cff file using the cffconvert GitHub Action. That way, it's a little bit easier to be robust against future changes to the CITATION.cff file.

BTW it's perfectly fine if you don't feel like accepting this Pull Request for whatever reason -- we just thought it might be helpful is all.

We found your repository using a partially automated workflow; if you have any questions about that, feel free to create an issue over at https://github.com/cffbots/filtering/issues/

On behalf of the cffbots team, @abelsiqueira / @fdiblen / @jspaaks

[hechth]

@florian-huber