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

[tim-band]

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.