support for annotations that break the assumption of one key:value pair per cell (BICAN)

Examples:
1. A single neuron may have signal via multiple neurotransmitters.
2. Encoding a dendrogram (single inheritance hierarchical clustering of cell sets with no fixed levels - also potentially confidence scores for branches).
  - Potential implementation:
  - Simply allowing a single cell set to have multiple parents allows us to record this.
Implications:
1. Current schema explicitly rules this out : "The unique name of the set of cell annotations. \nEach cell within the AnnData/Seurat file MUST be associated with _a 'celllabel' value in order for this to be a valid 'cellannotation_set'." Note the singular here https://github.com/cellannotation/cell-annotation-schema/blob/main/general_schema.json#L43
2. It is useful to keep the assumption of the current schema to term sets where it applies. I suggest that we assume it applies unless explicitly stated that it doesn't in the cell_annotation_key metadata (see #2 #3)
3. We need to think though how this could work with flattened representation in anndata
  - Option 1: store as lists in obs values - anndata supports this as the e.g. NT: ['Glu', 'Gly']
  - Option 2: concatenate labels in alphanumeric order with some standard delimiter e.g. NT: "Glu-Gly"

cellannotation / cell-annotation-schema