Parent group entities to object entities on the OBJ loader

[carolhmj]

Related forum issue: https://forum.babylonjs.com/t/obj-export-to-babylon-nodes-are-doubled/46028
The cause for the apparent doubling of nodes was having an "o" entity followed by a "g" entity:

In our loader, encountering both of these definitions results in creating a new mesh. However, groups are parts of objects, so we should be parenting the group to the object to make that relationship explicit.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
refs/pull/14565/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14565/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14565/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/14565/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/14565/merge
https://gui.babylonjs.com/?snapshot=refs/pull/14565/merge
https://nme.babylonjs.com/?snapshot=refs/pull/14565/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/14565/merge#BCU1XR#0

[RaananW]

Apart from the logging - LGTM. approved, make sure to resolve the logging comment before merging.