Compose embedding when returning it

See: https://github.com/dwavesystems/dwave-inspector/issues/39.

When `return_embedding=True` (on embedding composite sample call), only the top-level embedding is returned, not the composed embedding of all children and the parent.

Start with the linked issue, dwavesystems/dwave-inspector#39, and trace the embedding composite sample call when return_embedding=True. Verify how the returned embedding is assembled; done means it includes the composed embeddings for all children and the parent, rather than only the top-level embedding.

Written by the indexing model from the issue text.