[wip/{py-,}example-robot-data] Update to v3.1.1

Changes since v3.1.0:
- add path.py.in, similar to path.hpp.in
- romeo_description: remove unused files
- anymal_b_simple_description: Add missing inertia, fix for simulator
- hyq_description: Add inertia information for simulator
- [Python] switch to numpy arrays