Metadata about model features

So various model primitives might need to expose some additional metadata. One aspect of metadata is which task subtypes a model supports (what is currently in the problem schema as task_subtype). I suggest we add a top-level field model_features with:

"model_features": {
  "types": ["BINARY", "MULTICLASS", "MULTILABEL", "UNIVARIATE", "MULTIVARIATE", "OVERLAPPING", "NONOVERLAPPING"]
}

So types would be a list of enumeration. This (together with primitive family) would then help with some simple matching from task type and subtype to relevant primitives. In the future we could also add more other features describing models.