Support 3DTiles styling of Cesium

[valentinMachado]

Cesium propose a styling for the 3DTiles ref
It could be nice to support it in itowns. Maybe by creating a parser which would convert these styling.json files into itowns.Style objects.

[alexLuky]

I think first we need to render in same quality as Cesium does. Here is a link to another discussion about this.