Cesium 3dTtiles generation

[kiselev-dv]

This is a draft PR

Available configuration options

• writeTilesetJson adds <out-file>.tileset.json for gltf output with region and transform
• subdivideTiles outputs 2 gltf files, <out-file> with top 100 biggest meshes and <out-file>_0 with the rest of meshes

There is also console.TilesetPyramide class for generation of tileset trees out of *.tileset.json files

TODO

• Account for separate location for *.tileset.json files, as well as for different web roots
• Allow to subdivide tiles further (mb add one more level), account for geometricError
• Make number of meshes in subdivisions configurable
• Make spatial subdivision (subdivide tiles as quad tree first, then further subdivide by mesh type and buildings size)