[viewer.py] unloaded stage cycling

[aclegg3]

Motivation and Context

Add feature to viewer.py for cycling through unloaded stages in a SceneDataset via SHIFT-TAB keypress.

Currently, scenes will not be created for stage files without separate configs unless explicitly set as the scene_id. viewer.py scene cycling with TAB cycles registered scene instances, so unregistered stages will never be loaded. This feature adds a way to force new stages to be loaded with cycling to make investigation of SceneDatasets such as gibson and MP3D a bit easier.

How Has This Been Tested

Locally w/ gibson

Types of changes

• [ ] 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

• [ ] 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.
• [x] All new and existing tests passed.

[aclegg3]

Note: considering refactor to use same approach as Viewer.cpp: construct a list of scene/stage handles in advance and then cycle through that list.