Add table for defining cellannotation_set_metadata

[dosumis]

This allows for descriptions + rank

As currently defined:

  "cellannotation_setname_metadata": {
      "properties": {
        "name": {
          "type": "string",
          "description": "name of annotation key"
        },
        "description": {
          "type": "string",
          "description": "Some text describing what types of cell annotation this annotation key is used to record"
        },
        "rank": {
          "type": "integer",
          "description": "A number indicating relative granularity with 0 being the most specific.  Use this where a single dataset has multiple keys that are used consistently to record annotations and different levels of granularity."
        }

Related #23 #22

• Notes - not happy with name of this object + work still needed in original schema to fold into top level properties.