ObjectReferences work with ObjectPlacer

[cvolkcvolk]

Summary

ObjectReference can be used in object placement (as Anchor Objects).

• Provide ObjectReference with the required fields for it to be able to be used in the Object Placement

• ObjectReference points to a sub-prim inside the parent's USD, not a separate USD file. Adds a function compute_local_bounding_box_from_prim() to get the bbox of the specific sub-prim &
  apply the parent asset's scale to the resulting bounding box.

• Helper function to center, translate and scale AxisAlignedBoundingBox, removing some repetitive code.

• Moved following functions to ObjectBase as they can be shared among Object and ObjectReference.

    def get_relations(self) -> list[RelationBase]:
    def get_spatial_relations(self) -> list[RelationBase]:

[viiik-inside]

Looks amazing. Just some minor suggestions from my side.

[alexmillane]

This looks great! Thanks for doing that!