Replace sickle with xmltodict

@jgrieb Please check if code/mapping is ok that way (some are marked with TODOs). Also the pipeline still fails on the last step (target_handler.create/update) which needs to be fixed

[@awellmann-lrz I changed the target branch, from now on all new developments are first merged into the dev branch and if the harvesting works fine in the staging environment we merge from dev into main

Closes Issue #51 (closed)