Support nested choice labels

[mikix]

This is something that @Dtphelan1 flagged.

Label Studio can set up Choices that are nested below a Label. So maybe you label "Fever" and then have a sub choice of "Mild", "Severe", "Deadly" or something.

Thoughts around supporting this:

• Label studio does not enforce that a choice is filled in.
• How to get choice config? Just have the use specify options in the config.yaml, like they do for labels? Maybe re-use grouped-label structure?
• We'd have to parse the Choices in the export - they are a separate kind of object next to a label (they point at the label, share the same "id" value, and use the same span)
• How to specify on the CLI whether you want math run the leaf choices vs the roll-up (choices vs labels) - both of which the user might want - could show both by default, but what about when using the --csv flag?
• The accuracy math will need changes to consider all choices together (like, one FN/FP, not 3 for for the Fever case)