DebugVisualizer: equirect, removal/deconstruction, image matrix util, and unit test

[aclegg3]

Motivation and Context

This PR adds a set of features to the DebugVisualizer along with a basic unit test.

• Adds Equirectangular mode: dbv.equirect=True or DebugVisualizer(sim, equirect=True) to use an equirectangular sensor
• Adds dbv.remove_dbv_agent() - clean up after dbv by removing the added agent/sensor. This is now also called on deconstruction.
• Adds dbv.clear_color - set the background color of dbv images (must be set before initialization).
• Adds utility function stitch_image_matrix - stitches together a set of PIL Images into a single image matrix.

Example Equirect Image from DBV:

Can be viewed in 3D with applications like https://arachnoid.com/3DViewer/

Example stitched image matrix from a set of region omnidirectional renderings:

How Has This Been Tested

Locally + CI tests

Types of changes

• [Development] A pull request that add new features to the habitat-lab task and environment codebase. Development Pull Requests must be small (less that 500 lines of code change), have unit testing, very extensive documentation and examples. These are typically new tasks, environments, sensors, etc... The review process for these Pull Request is longer because these changes will be maintained by our core team of developers, so make sure your changes are easy to understand!

Checklist

• [x] My code follows the code style of this project.
• [x] I have updated the documentation if required.
• [x] I have read the CONTRIBUTING document.
• [x] I have completed my CLA (see CONTRIBUTING)
• [x] I have added tests to cover my changes if required.