Raise exception if root link is missing in URDF

[lorycontixd]

When no link named 'root' is present in the urdf loaded by mujoco, the following exception is automatically raised:

Traceback (most recent call last):
  File "/home/lconti-iit.local/Documents/elements/element_lower-leg-morphology-optimization/src/passive_walker/CMDWalker/minimal_sim.py", line 52, in <module>
    mujoco_instance.load_model(
  File "/home/lconti-iit.local/Documents/ami-forks/comodo/src/comodo/mujocoSimulator/mujocoSimulator.py", line 59, in load_model
    mujoco_xml = robot_model.get_mujoco_model(floor_opts=floor_opts, save_mjc_xml=False)
  File "/home/lconti-iit.local/Documents/ami-forks/comodo/src/comodo/robotModel/robotModel.py", line 367, in get_mujoco_model
    mujoco_model = mujoco.MjModel.from_xml_string(urdf_string)
ValueError: XML Error: URDF joint parent or child missing
Element 'joint', line 21

I think this error is missleading as it points towards a missing joint instead of a missing root link.

Therefore, I think it'd be best to search for a link named root as the model loads, and raise a custom exception if this link is not found.

[lorycontixd]

Resolving lines:

has_root_link = "root" in [l.attrib['name'] for l in root.findall(".//link")]
if not has_root_link:
    raise ValueError("The URDF file must have a link named 'root'")

[lorycontixd]

Reference the second commit, not the first one