--Define constant and bindings for stage ID

[jturner65]

Motivation and Context

The stage ID is referenced in python and from lab in numerous places, usually as a mark of where added objects' ids begin, or else as a final target for collision detection. Unfortunately, this is often referenced or checked by using magic number references to the stage (i.e. -1).

This PR introduces a global constant to denote the stage ID (esp::RIGID_STAGE_ID), and bindings to that constant to expose it to python (habitat_sim.stage_id), which should be used to refer to the stage ID. A matching Habitat-Lab PR is pending for this support as well, once this PR is merged. Ultimately this is to facilitate the instanced -based semantic sensor support PR here

How Has This Been Tested

All c++ and python tests pass locally

Types of changes

• [x] Docs change / refactoring / dependency upgrade
• [ ] 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)
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.