Add asserts for mj_id2name calls in mujoco_utils

[DavidPL1]

Description

Added asserts for calls to mj_id2name calls. In case the resource is not available in the model definition the call returns -1. After these calls this case is not handled as an error and values for the wrong joint, site, or body are returned.

This change increases the reusability of mujoco_utils and is especially helpful when creating or customizing mujoco environments.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
  (not necessary here)
• I have made corresponding changes to the documentation
  (not necessary here)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
  (not necessary here)
• New and existing unit tests pass locally with my changes

[Kallinteris-Andreas]

for additional refernce:
https://mujoco.readthedocs.io/en/stable/APIreference/APIfunctions.html#mj-name2id