PartitionConfig

Language-independent config for splitting a dataset into a training set and a held-out test set. A first-class, auditable operation -- the same reason `ingest` is one -- rather than a field on `SuperConfig`: how a held-out set was produced is a decision the record must be able to report. `method` selects the variant and its settings are siblings of it, validated against the variant's own schema below. Anything not set takes rtemis's default, so an object holding only `method` is that variant with every default.