Cherry-pick nested model support to sdf10#338

Merged

scpeters merged 1 commit intogazebosim:sdf10from scpeters:pick_nested_model_dom_10

Aug 17, 2020

+568-65

Commits on Aug 17, 2020

[sdf9] Support nested models in DOM and frame semantics (gazebosim#316 )
scpeters
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