gltf: parent node for an element and the representations as children

[katehryhorenko]

BACKGROUND:

• When I added multiple representation part of the functionality was broken because we are assuming in Explore that the first 3D object is a parent object and it's the element geometry. But with multiple representations it can be one of the representations

DESCRIPTION: -This PR changes the gltf serialization for multiple representations, The representation instance becomes the child node of the element node. Element node contains Transform.

TESTING:

REQUIRED:

• [x] All changes are up to date in CHANGELOG.md.

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This change is