-- Articulated Object creation restructuring

Motivation and Context

This PR refactors the AO creation process so that configuration-based modifications to an AO do not end up applying or being attached in any way with the URDF-built Model, but rather are applied/attached to the resultant BulletArticulatedObject.

This is required because multiple BulletArticulatedObjects can have their own attributes/settings/user_defined configurations despite having been built by the same URDF/Model. With this PR, the Model will remain a static reflection of the data specified in the source URDF, while the BulletArticulatedObject built from this model will manage its own customizations of the original model as specified in the ArticulatedObjectAttributes.

This PR also addresses the bug referenced in this Lab issue.

Note : This PR as it stands still retains geometric and mass scaling in the Model.

How Has This Been Tested

Currently C++ tests pass

Types of changes

[ ] Docs change / refactoring / dependency upgrade
[x] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

[x] My code follows the code style of this project.
[x] My change requires a change to the documentation.
[x] I have updated the documentation accordingly.
[x] I have read the CONTRIBUTING document.
[x] I have completed my CLA (see CONTRIBUTING)
[ ] I have added tests to cover my changes.
[x] All new and existing tests passed.

facebookresearch / habitat-sim