Convert tileset `properties` to `statistics`

[j9liu]

The tileset.json schema has a field for properties which convey the minimum / maximum values for features across an entire tileset. Since then, properties has been deprecated and essentially replaced by tileset statistics.

Currently, upgrading with --targetVersion 1.1 leaves the properties untouched. It would be great to automatically upgrade this object to a statistics one, though it's not entirely straightforward. You'd need to create a metadata schema that the statistics are based on, and then translate the properties into appropriate class property types that match the content in the glTFs. But it's certainly doable.

[javagl]

That would certainly be desirable and in line with the deprecation of the properties. Generting a schema might not even be soooo difficult. The more difficult part is the question what happens when the tileset already does have a schema (opening the whole can of worms of CesiumGS/3d-tiles#802 ).

But maybe it can be narrowed down sensibly, for the (probably most common) case where the input indeed is a 1.0 tileset that does not have any metadata yet anyhow (and this might then actually be a low-hanging fruit).