Add Floating Joint Support

[Levi-Armstrong]

There has been some recent discussion on adding floating joint support which should be fairly straight forward, but first I think it would be good to discuss the current use cases. @marip8 this recent came up in our conversation, but can you describe a current use case? If the intent is to mostly treat these as fixed joints with periodic updates and not be considered like active joints for the purpose of computing jacobian of the scene, then this should be fairly straightforward. If you want to be able to compute the jacobian relative to changing this floating joint then that would likely need to follow the approach used in the open PR for adding the planar joint.

[marip8]

The primary use case that I have in mind is for adding a link (with or without geometry) whose pose would be computed by sensor data (e.g., dynamic collision object, AprilTag, (inverse) odometry frame for a mobile manipulator, etc.).

If the intent is to mostly treat these as fixed joints with periodic updates and not be considered like active joints for the purpose of computing jacobian of the scene, then this should be fairly straightforward.

I think this would be the case for the use-cases I listed above. I wonder if this should this be treated any differently than a fixed joint though, provided we expose a convenient hook for changing the pose of the link. We can currently do this by applying a MoveJoint environment command, but I'm not sure what sort of overhead is incurred by doing that since that is constantly replacing the joint in the scene graph (I presume). Maybe a floating joint could just be a fixed joint that has the ability to change the joint pose more directly