vtkCesium3DTilesReader improvements

Extend 3D Tiles reader to:

• allow B3DM tiles

• support external tilesets

• enable applying textures

• fix a bug where only the first partition in a tile was loaded

• BREAKING_CHANGE: the reader produces a vtkPartitionedDataSetCollection instead of a vtkPartitionedDataSet.

These changes require the following vtkGLTFReader improvements:

• using the GLTF reader to read an embedded GLB inside another file (the B3DM in this case)

• adding an option to GLTF to produce double points - this is needed for 3D Tiles

• fixing the way textures are exported so that the API wrapped in python