Added LODGroup support

[hybridherbst]

Adds support for these kinds of scenarios that commonly arise in complex scenes and basically always arise in photogrammetry scenes with chunked meshes:

[Fewes]

Looks like a decent addition! However, this makes it impossible to bake for a selected LOD other than 0 which can be useful if different LODs use different textures. Could you add a check if LOD0 is present in the selection before doing the discarding, or better yet only discard LODs which share materials?

[hybridherbst]

OK, adjusted to check for materials instead.
Note that while this enables to manually bake for specific LOD levels, it won't really work for baking for multiple LOD levels at the same time - you would have to adjust your baking logic to make sure to always just have the LOD you want to bake on at the same time, otherwise there's heavy AO between LOD groups.
I'd consider that case arcance enough to not be covered here, to be honest. A note about it in the readme should suffice.