fix ellipsometry example from workshop updates
This MR fixes the ellipsometry example, as the NXoptical_spectroscopy definition was updated a few weeks ago and therefore as well influenced the ellipsometry example (as it depends on NXoptical_spectroscopy). Mostly this covers the changes in the .json, archive.yaml and eln_data.yaml file.
In addition, this MR fixes the new pluginization of NeXus data converter in nomad. (default.yaml & values.yaml and removal of ".nxdl" from .json and archive.yaml file)