Optionally ignore repeated assets in mjSpec attach

[Balint-H]

Hello!

I'm working with procedural scene editing with mjSpec (in mujoco 3.3.7). I'm enjoying the process mostly, but there are still some pain points. I'm still learning, so please let me know if I'm making mistakes or missing some obvious alternative.

One in particular application of mjSpec is duplicating subtrees that contain assets (meshes, materials, etc). For example, if the scene had one robot with assets, adding extra arms to the robot or extra robot copies in the scene. I achieve this by creating a copy of the spec, finding the relevant body, and attaching it to the original spec.

Attaching a copy this way leads to duplicate mesh_name errors (as the copy's meshes are trying to be added to the original spec). This necessitates renaming duplicate meshes (which wastes memory), or deleting these assets from the spec copy prior to attaching. Similar issues are present with keyframes, materials, textures, sensors, actuators (even if using prefixes during the attach, which seemingly only works on bodies, geoms and joints).

As attaching a kinematic tree with assets that can be reused across copies is a useful operation, could we have an optional argument in the attach function to assist with this? It can signal to ignore and not raise an exception if an asset with the same name is attempted to be attached to a spec, and assume the attached subtree's named references refer to the ones already present in the target spec.

Thanks!

[yuvaltassa]

cc @quagla @havess