Add empty visual box to all virtual links

[stephane-caron]

• Introduces a virtual_link macro that supersedes the former virtual_link_inertia
• Addresses the issue identified by @Tordjx in Added camera_eye virtual frame #19 (comment)
• Fixes IMU frame orientation does not render properly #20

[stephane-caron]

Checking that the IMU frame orientation is now displayed correctly:

python display_frames.py --only imu

We can check from the pi3hat docs that this is indeed the proper orientation:

• The pi3hat is mounted with USB ports to the left of the robot (NB: when looking at the model, battery is at the back) and with the pi3hat below the raspi with respect to the vertical
• The y-axis (green) is indeed pointing towards the left of the robot (= USB ports)
• The z-axis (blue) is indeed pointing down
• The x-axis (red) points towards the back (battery), which I checked visually is indeed where the CAN connectors are

As to the position, we indeed haven't set it properly in the description, but that's a different issue 😉