GRIFFEL_bim_data.schema.json is an invalid json schema

[teddemunnik]

GRIFFEL_bim_data.schema.json appears to be an invalid json shema.

The item property in properties/nodeProperties/items/properties/properties should be items.

[javagl]

I think that it's not invalid in the strictest sense: The item in this case just counted as an additional property (and arbitrary ones are allowed by draft-04 by default).

But it clearly was the intention to make sure that the properties array in the JSON itself (not the JSON schema, but the JSON) was supposed to only contain IDs (integer values). (Right now, I think that the schema would allow arbitrary objects...)

PR in #2375