Automatically selecting the only label in single-label segmentation tasks

[leonoverweel]

I'm currently using UDT for a single-class segmentation task: person detection, where the only bounding box(es), if any, I draw on an image, are all from the same class (person). It'd save a few clicks if UDT knew to automatically select the only label there is in this situation.

(This feature request overlaps a bit with #289.)

[seveibar]

Great idea! Should be easy to do, thanks for the writeup!!