Importing a tile should group all the objects neatly into collections/nested objects

[space928]

Describe the bug

As it stands, o3d-io is a bit random in how it imports entire tiles with objects being split across multiple LOD collections which are somewhat randomly enabled or disabled.

Ideally the imported structure should look more like:

(C) tile_-1_-1
    (C) house_a.sco.0001
        (M) house_a_building
        (M) house_a_fence
     (C) house_a.sco.0002
        ...

As such the tile collection can store .map global properties, each collection for .scos can store sco globals, etc...

In this proposal, LODs simply won't be imported for entire tiles. I don't think they're necessary for this use case.