Feature: Export Detection Coordinates

[I-AM-EXFOLIATING]

It would be useful for integration with other tools, and for diagnostic purposes, to export the image coordinates of detections to a text file.

My use-case is exporting AprilTag centers for a non-kalibr calibration tool which does not support AprilTags directly, but can consume a text file containing the detection information.

I initially tried doing this with the apriltags_demo from https://people.csail.mit.edu/kaess/apriltags/ however I found that the AprilTag detector in kalibr is exceptionally robust compared to the "official" detector which did not seem to like my Aprilgrid generated by kalibr (I suspect the small black squares on the corners confuse the outline detection). Is there some special sauce in kalibr's implementation?

If this feature is implemented, the exported data must contain the timestamp for the originating frame (seconds + nanoseconds) and the coordinates of the detections (center of tags in the case of AprilGrids).
I've tried to hack this in myself to test if the feature could be viable, but ran into issues of both timestamp and detection coordinates being available in the same scope, so it would require some refactoring.