Match frame of UR robots with base frame used by real robot controlbox

[tlpss]

This PR changes the orientation of the base body of the UR robots to match the frame of the robot with the frame used by a real robot controlbox. See the original URDF file for reference, where they also added a separate link that corresponds to the real robot base frame.

This ensures that a given joint configuration corresponds to the same pose for a real robot (expressed in the base frame) and a simulated robot (expressed in the 'attachment site' / world frame). This avoids confusion and allows to use real world trajectories/IK tools/... in the simulator.

This PR closes #75.

One thing to note is that there is currently no site/body within the robot model that corresponds to the actual base frame. I'm not sure if there is a convention for this within the repository? (@kevinzakka).

Following code snippet can be used to check the poses corresponding to a joint configuration match for a real robot and simulated robot:

    import robot_descriptions
   from dm_control import mjcf

    robot = robot_descriptions.ur5e_mj_description.MJCF_PATH
    robot = mjcf.from_path(robot)
    physics = mjcf.Physics.from_mjcf_model(robot)

    joint_config = np.zeros(6)
    physics.bind(robot.find_all("joint")).qpos = joint_config
    
    # get attachment site pose
    attachment_site = robot.find("site", "attachment_site")
    attachment_site_pose = physics.bind(attachment_site).xpos
    attachment_site_orientation = physics.bind(attachment_site).xmat.reshape(3,3)
    pose = np.eye(4)
    pose[:3, :3] = attachment_site_orientation
    pose[:3, 3] = attachment_site_pose
    print(f"attachment site pose: \r\n {pose}")

    # get real robot pose for same joint config
    # ur_analytic_ik has been validated to match the base frame used by real robot controlboxes
    import ur_analytic_ik
    FK_pose = ur_analytic_ik.ur5e.forward_kinematics(*joint_config)
    print(f"forward kinematics pose: \r\n {FK_pose}")

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[tlpss]

@kevinzakka , I have signed the CLA
Is there anything else blocking this PR?