Merge nearby detected boxes. Add unit tests.

[iuliaturc]

Here's an example of why this is useful.

green = boxes detected by Azure blue = merge of the detected boxes red = manual annotation

After merging the boxes, it's a lot easier to compare blue against red.

[mihail911]

hmmm i honestly could see it going multiple ways. namely what is the manual annotation also has multiple boxes? then in principle you could use different heuristics to compare them.