Add simulator physics collision cache.

[0mdc]

Motivation and Context

This adds a cache for the habitat-sim get_physics_contact_points() function.

This function is called multiple times a frame and is quite expensive.

On a reference single-learn HITL application, this improves the framerate from 11.3 to 13.8 FPS.

How Has This Been Tested

Tested locally + unit tests.

Types of changes

• [Optimization]

Checklist

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

[aclegg3]

Let's add this as a member variable and set it to dirty on reset/reconfigure also. Those events will both reset time and change the scene state.

[aclegg3]

My other concern here is that moving objects will invalidate the cache and any calls to Simulator.perform_discrete_collision_detection will be missed.
Perhaps you want to override that function also to modify the dirty flag.
I think at that point the likelihood of erroneous contact point data should be minimized.

[0mdc]

My other concern here is that moving objects will invalidate the cache and any calls to Simulator.perform_discrete_collision_detection will be missed.

Shouldn't this function change the world time?

No, the role of this function is to run the collision detection procedure one time to update the internal caches which we query when calling the contact point getter. No physics are done by that function, so world time shouldn't change.