Update dataset schema

[jameshadfield]

As part of the multi-tree / exploded-tree functionality introduced in nextstrain/auspice#1442 the next auspice release will allow an array of trees and the schema needs to be updated accordingly.

Current schema (abridged):

"tree": {
    "type" : "object",
    "$comment": "The phylogeny in a nested JSON structure",
    "properties": {
        "children": {
            "type": "array",
            "minItems": 1,
            "items": {"$ref": "#/properties/tree"}
        }
    }
}

The new schema is similar, except that the tree property can now be an object (as currently) or an array of objects. In the latter, each object is the same as the current "$ref": "#/properties/tree".

It is fully backwards compatible and doesn't need a version bump.

An example dataset is included (which currently fails validation)

tree-array-example.json.gz

[tsibley]

I was already looking at the schemas this morning when I saw this issue, so I drafted a change for this in #851.