Replace sets with set_names in CreateDataset call

Starting with Arkindex 1.6.0, the sets field in CreateDataset becomes read-only, as it is no longer a list of set names but a list of dataset sets, with an ID and a name. The set_names can be used instead, which creates DatasetSets while creating the dataset.

This field is used by arkindex elements ml-splits, so this command needs to be updated.