BICAN specific fields

[dosumis]

BICAN needs:

• 2 #3

• support for hierarchies of cell sets built on a set of fixed clusters (most granular cell sets). Rather than encoding this as sets of cell IDs in each annotation object, this should be stored in the form of parent-child relationships store in these objects - see https://github.com/cellannotation/cell-annotation-schema/blob/main/user_stories.md#bican
  • [x] #5
• [ ] #6