Fix Aliengo URDF source

[Danfoa]

This PR updates the URDF configuration for the aliengo robot by switching to the correct source provided by unitree. The previous URDF file erroneously included two floating base joints (root_joint and floating_base), effectively introducing an unwarranted 7 degrees of freedom (DoF) for each joint.

The issue is evidenced by the following debug output, which highlights the duplication of the JointModelFreeFlyer type:

[...]
[DEBUG][morpho_symm.robots.PinSimWrapper] Loaded pinocchio robot model for aliengo_mj. The robot's joints are:
[DEBUG][morpho_symm.robots.PinSimWrapper] 	 -0   universe             joint_type: JointModelRX         nq:1   nv:1   idx_q:-1  idx_v:-1 
[DEBUG][morpho_symm.robots.PinSimWrapper] 	 -1   root_joint           joint_type: JointModelFreeFlyer  nq:7   nv:6   idx_q:0   idx_v:0  
[DEBUG][morpho_symm.robots.PinSimWrapper] 	 -2   floating_base        joint_type: JointModelFreeFlyer  nq:7   nv:6   idx_q:7   idx_v:6  
[DEBUG][morpho_symm.robots.PinSimWrapper] 	 -3   FL_hip_joint         joint_type: JointModelRX         nq:1   nv:1   idx_q:14  idx_v:12 
[DEBUG][morpho_symm.robots.PinSimWrapper] 	 -4   FL_thigh_joint       joint_type: JointModelRUBY       nq:2   nv:1   idx_q:15  idx_v:13 
[...]

[stephane-caron]

Thank you for this proposal 👍

The change LGTM. The failing test reports this error (probably a unit test to update, I haven't looked at it deeper):

2024-04-14T18:49:28.8898940Z Traceback (most recent call last):
2024-04-14T18:49:28.8907670Z   File "/home/runner/work/robot_descriptions.py/robot_descriptions.py/tests/test_descriptions.py", line 34, in test_all_descriptions
2024-04-14T18:49:28.8908794Z     self.assertTrue(hasattr(description, "MJCF_PATH"))
2024-04-14T18:49:28.8909292Z AssertionError: False is not true

[stephane-caron]

There were a couple more changes needed (updating _descriptions.py, readme and changelog), but otherwise this PR was good to go.

I could not push to your branch (there is a box to tick somewhere when opening a PR to allow project maintainers to push), so I went ahead and merged the PR in 1ef00c3 after adding the fixes and running the CI locally.

Thank you for this contribution!