describing ocrd-tool.json parameter object properties, rev'std

[bertsky]

Since #206 we can moderately restrict an ocrd-tool parameter of type object's properties and even set additionalProperties:

spec/ocrd_tool.schema.yml

Lines 95 to 100 in 506b339

	properties:
	type: object
	description: Describe the properties of an object value
	additionalProperties:
	type: boolean
	description: Whether an object value may contain properties not explicitly defined

However, according to the JSON data schema, additionalProperties is either false or an object (i.e. dict) itself.

For example, in ocrd-kraken-recognize I'd like to have:

        "blla_classes": {
          "description": "Class mapping for the region types trained into blla_model.",
          "type": "object",
          "minProperties": 2,
          "additionalProperties": { "type": "string",
                                    "enum": ["TextRegion", "ImageRegion", "LineDrawingRegion",
                                             "GraphicRegion", "TableRegion", "ChartRegion",
                                             "MapRegion", "SeparatorRegion", "MathsRegion",
                                             "ChemRegion", "MusicRegion", "AdvertRegion",
                                             "NoiseRegion", "UnknownRegion", "CustomRegion"] },
          "default": {"text": "TextRegion", "image": "ImageRegion", "line drawing": "LineDrawingRegion",
                      "graphic": "GraphicRegion", "table": "TableRegion", "chart": "ChartRegion",
                      "map": "MapRegion", "separator": "SeparatorRegion", "maths": "MathsRegion",
                      "chem": "ChemRegion", "music": "MusicRegion", "advert": "AdvertRegion",
                      "noise": "NoiseRegion", "unknown": "UnknownRegion", "custom": "CustomRegion"}

That is: allow only certain types of properties (namely all PAGE-XML region type names).

This is currently not possible to express.

Also, minProperties and maxProperties is missing entirely.