Enable camera frames in URDF

[mmattamala]

This PR adds the camera frames as fixed frames in the xacro files. Since Spot actually publishes the TFs for the cameras (but at frame rate, as part of the camera publication), we obtained the fixed transformations from a rosbag and confirmed they didn't change over time.

The changes include:

• A flag static_camera_frames was added to the xacro, which can enable this feature
• The launchfiles were updated accordingly to support this flag:
  • spot_description/launch/description.launch
  • spot_driver/launch/driver.launch
  • spot_viz/launch/view_model.launch
• The driver spot_driver/scripts/spot_ros.py also was modified to disable the publication of TF at frame rate when the flag is enabled.
• Lastly, I also added spot_description/launch/load.launch to load the model into the parameter server, which is useful when using rosbags.

I tested the flag using view_model.launch, and also lauching the driver in my laptop and checking that the static transforms were being published when the flag was enabled (even when Spot was not connected).

@heuristicus Please test it as we discussed today. Tell me if you need me to modify anything or feel free to do it if it's easier.

Flag disabled:

Flag enabled:

[heuristicus]

I fixed the if statements and also modified the getImageMsg function to make it so that the whole tf computation step is skipped if None is provided as the frame. It probably doesn't make much difference in terms of computation but better to not do things that aren't necessary.

Dynamic frames:

Static frames:

But with these changes for some reason I can't see the depth clouds any more, will test more and make another commit to fix.

[mmattamala]

Nice, it was a minor change fortunately.