Compatiblity of "difficulty" values in metadata

At the moment, the sharing platform accepts values simple, medium, and advanced for the difficulty of a resource. The wizard developed at TU Wien uses very easy, easy, medium, difficult, very difficult in English and sehr leicht, leicht, mittel, schwer, sehr schwer in German. These values are not accepted at the moment, so exercises using them do not show up on the sharing platform. This should change.

Considerations

Consistency of Systems

Ideally we would only use a single system. However, breaking valid metadata files is undesirable, so it would probably work best if we deprecate one system but support it for now, offering a way to automatically convert existing metadata files before we drop support.

Consistency of Language

Working with a single language in the metadata and doing translations only in the front-end keeps the system simpler and more consistent. I suggest keeping the values for which we have a vocabulary in English. Especially with a wizard for the metadata, the user might not even see the metadata file.

Compatibility with Artemis

Artemis distinguishes three difficulty levels. At the moment we use a 1-to-1 mapping, but this could be changed such that we use a mapping like

{very easy, easy} -> easy, medium -> medium, {difficult, very difficult} -> hard

for import to Artemis and

easy -> easy, medium -> medium, hard -> difficult

for exporting from Artemis.