[REQ] \<site\> tag support Mujoco

[ped02]

Description

<site> tag support for Mujoco. Used to query location/orientation attached to a body/shape wrt to some point in space. I'm not sure if theres already a native way to do this in warp or not.

Location (import_mjcf.py)

I tried implementing a proof of concept by adding body (with mass = 0, I_m = 0, etc. in order to not mess with dynamics, but I'm not sure if this would cause instability or not), then attach the site body using fixed joint to the parent's body. Then shape is added to the site body, and collision for the shape is disabled.

For backward compatibility, can provide optional kwarg to enable site with default being false.

Edit 1 (31 Jan 2025):
Maybe a helper function to query body index of the site by name as well.

Context

I was looking at myosuite and they use tendons mainly for control and sites for querying/sensing location, etc. For my use case, I used site to track location of a certain part of the articulation chain to calculate control error.