pass back corner detections

[inventor2525]

Just wondering if there's interest in this. Possibly made optional.

Reasons we wanted the change:

1. To try removing rotational ambiguity in limited instances using extra points, possibly inner corners
2. Track tag looking for blur resistant extra markers (possibly until ArucoDCF [not the current ros node]).
3. Add other adjacent markers in the future (color calibration).

[christian-rauch]

I guess publishing the corner coordinates too could be useful. I did this in the ROS2 version of the node: https://github.com/christianrauch/apriltag_ros. However, I recommend using standard messages for this instead of raw lists of concatenated coordinates.

So far, this would be useful for debugging and visualisation. I don't follow how you want to use them in the applications you listed. Most of them will require changes to the library itself, not this ROS node.

[julled]

I would also interested in receiving the corner and center detections as e.g. Detection2D messages