refactoring around model directory makes examples not working

[HiroIshida]

after this https://github.com/iory/scikit-robot/pull/174, example/signed_distance_function.py example/trimesh_scene_viewer.py example/pybullet_robot_interface.py are no longer be working, because they all import primitive from models.primitives.

I think maybe running all examples in the github action to check them working is a good practice. But also this makes test slower a lot.

[iory]

Thank you for your error reports. I created a PR to fix the errors.

I think maybe running all examples in the github action to check them working is a good practice. But also this makes test slower a lot.

It seems good. However, those examples use GUI. It will take a little bit of time to cope with them. All users can feel free to report an issue!