Zenodo harvester

@JohnDOEbug and @awellmann-lrz I updated the code in your branch to make it work with the latest approach based on the LinkML-definition of the schema.

Please try out on your machines with kh_populator populate-kh -p harvest_zenodo -t local_rdf_file and check whether it works as expected, if yes give me a signal here so we can merge.

There some minor problems but I would like to merge into main first and will then open up a new issue for those.

Merge is related to #26