Fix error when resolving possible objects to pick while some are carried by other robots

[robin-mueller]

When picking objects with multiple robots in a world, it is likely, that some are currently carried and do not reside on locations anymore. Since Robot doesn't have a parent attribute, an error is raised when checking each object's location.

[world.py-1] [ERROR] [1734861471.897019297] [pyrobosim]: Error raised in execute callback: 'Robot' object has no attribute 'parent'
[world.py-1] Traceback (most recent call last):
[world.py-1]   File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/action/server.py", line 333, in _execute_goal
[world.py-1]     execute_result = await await_or_execute(execute_callback, goal_handle)
[world.py-1]   File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/executors.py", line 107, in await_or_execute
[world.py-1]     return callback(*args)
[world.py-1]   File "/home/robin/Desktop/px4-ros2-env/install/auto_apms_simulation/local/lib/python3.10/dist-packages/pyrobosim_ros/ros_interface.py", line 418, in action_callback
[world.py-1]     execution_result = robot.execute_action(robot_action)
[world.py-1]   File "/home/robin/Desktop/px4-ros2-env/install/auto_apms_simulation/local/lib/python3.10/dist-packages/pyrobosim/core/robot.py", line 917, in execute_action
[world.py-1]     result = self.pick_object(action.object, action.pose)
[world.py-1]   File "/home/robin/Desktop/px4-ros2-env/install/auto_apms_simulation/local/lib/python3.10/dist-packages/pyrobosim/core/robot.py", line 494, in pick_object
[world.py-1]     obj = query_to_entity(
[world.py-1]   File "/home/robin/Desktop/px4-ros2-env/install/auto_apms_simulation/local/lib/python3.10/dist-packages/pyrobosim/utils/knowledge.py", line 185, in query_to_entity
[world.py-1]     obj_candidate = resolve_to_object(
[world.py-1]   File "/home/robin/Desktop/px4-ros2-env/install/auto_apms_simulation/local/lib/python3.10/dist-packages/pyrobosim/utils/knowledge.py", line 339, in resolve_to_object
[world.py-1]     possible_objects = [
[world.py-1]   File "/home/robin/Desktop/px4-ros2-env/install/auto_apms_simulation/local/lib/python3.10/dist-packages/pyrobosim/utils/knowledge.py", line 345, in <listcomp>
[world.py-1]     or obj.parent.parent == location
[world.py-1] AttributeError: 'Robot' object has no attribute 'parent'

This PR fixes this issue by checking if the parent has an attribute called parent effectively verifying that it's not a robot. Consequently, objects can not be picked when they are carried by robots.

Note

I've also tried to verify using isinstance(obj.parent, Robot) but this requires to import Robot and causes a circular import error when done on top. It's also possible to import it inside the resolve_to_object method, but I guess this solution is sufficient.

[sea-bass]

This certainly works!

I also think this similar check would be good to add in the "room" case right above this block of code?

[sea-bass]

Thanks! I do agree there are better longer term solutions, but this definitely fixes the edge case for now.