Command `elements ml-split` should create Dataset

Closes #145 (closed)

Refs https://redmine.teklia.com/issues/5231:

arkindex elements ml-splits \
  --project f0dd8b4c-4479-447b-9e0c-8a6a12fc4bc3 \
  --uuid-list training_dataset.txt \
  --set test:0.2 val:0.2 train:0.6

Refs https://redmine.teklia.com/issues/5575:

• Remove socface script
• Do not use the selection
• Update the tests
• Update the documentation