--[BE] - Add utility function to specify whether object is ao or not

[jturner65]

Motivation and Context

This small PR exposes a read-only property ("is_articulated") in the python bindings that says whether an object is articulated or not, as an alternative to checking for the object type before attempting to access type-specific functions.

How Has This Been Tested

c++ PhysicsTest.cpp and SimTest.cpp and python test_physics.py have been expanded to test this field, and all tests pass locally.

Types of changes

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

Checklist

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

[aclegg3]

LGTM, thanks.
Should eliminate our frequent need for isinstance(ManagedArticulatedObject).