Create `energon upgrade` feature

[philipp-fischer]

The energon upgrade should upgrade an already-prepared dataset to a new version. That means

• .info.yaml will be upgraded to .info.json and the energon version will be added
• Sqlite databases will be created
• The tar index will be updated (*.tar.idx files)
• The user shall be warned if the number of shards or number of samples changed, because the upgrade method will not support that. In this case the user will need to reprepare

Maybe we can get away without touching every tar file to speed this up in some cases? To be investigated.