With null-partitioned grouped multivariate, need a way to specify that some numeric columns should be considered categorical

Either:
- allow `merge-categorical`
- propose `multivariate_choice` or similar; this doesn't allow for some numeric columns to be categorical and some to be numeric, though.
- allow some way to configure a column to be categorical in the `config.yaml`
- allow `orm.yaml` to set a column as categorical despite being numeric.
