Support hierarchies in VoxelModel assets

[RiscadoA]

Problem

Currently the VoxelModel asset supports only lists of grids in the same asset.
Some formats, i.e., .vox, support transform hierarchies within a single file. (#1409)
It is cumbersome to manually reconstruct the scenes when that information is already present in the file.

Solution

It would be nice to have that information also stored in the VoxelModel asset, so that, later on, we can make some tool to import full .cubos scenes automatically from a single .vox, for example.