Document mapping from MapMyCells JSON to CAS

[dosumis]

1. Summarise key information from doc https://portal.brain-map.org/explore/cell-type-references-and-algorithms
2. Find a small dataset on CellXGene with cortical cell types
3. Test this with MapMyCells + 3 different algos
4. Document the JSON
5. Make mappings to CAS

AIm for CAS. We need to be able to

1. Identify the taxonomy mapped to
2. Generate labelset metadata for annotation transfer labelsets
   
   https://github.com/cellannotation/cell-annotation-schema/blob/main/build/general_schema.md

• What can we get from the JSON? What do we need as args to some conversion method?
• What conventions do we need for naming labelsets

4. Generate labelsets
   • See this PR Cell level annotation transfer - BICAN_extension.json #115 for details of how to record confidence at the cell type level. See also https://github.com/cellannotation/cell-annotation-schema/blob/main/build/general_schema.md

Hierarchical mapping algorithms generate multiple mappings corresponding to multiple levels in the hierarchy. These should become multiple labelsets.