Adds body_pose_w and body_projected_gravity_b observations

[jtigue-bdai]

Description

This PR add body_pose_w and body_projected_gravity_b observation methods

Fixes # (issue)

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[Mayankm96]

Can we simply call it body_pose_w? What happens if user specifies a list of body ids? I think their flattned version should be given?

[Mayankm96]

Instead of link_name they must use body_names in asset config IMO.

[jtigue-bdai]

yeah body_pose_w is fine, and body_name is fine. I will switch to body vs link everywhere. (old habit to use link).

As far as the body_ids goes currently this observation will only produce one body driven by (link_name/body_name argument). if the users doesn't set the link name it will grab only the zeroth entry specified in the resolved SceneEntityCfg. So if they specify a list of body_ids they can only pick one of them.

I could see creating it in a way that all body_ids specified in the asset_cfg are provided in a flattened tensor, but this wasn't my original intention.

[Mayankm96]

I think flattening it is the most general way to go. If they want the "zeroth" body which is the root, then they should use the root_ alternative functions.

[jtigue-bdai]

Thats valid. I will rework it.