Allow users to customize dataset set names

• The arkindex elements ml-splits command creates an Arkindex dataset with the following sets: Train / Validation / Test
• DAN expects sets named with train / val / test
• These names are hard-coded, so data extraction will crash in this case
• We should allow users to customize these set names

Since we have a lot of parameters for data extraction, it could help to work on #148 in parallel.