Fix centroid imported from VIA file into bboxes dataset

[sfmig]

Is your feature request related to a problem? Please describe.
We talk about the centroid position of the bboxes in the docs, but what we are importing is actually the top left corner of the bounding box.

Describe the solution you'd like
Transform the imported data to store the actual bbox centroid in the position data array

centroid = top_left + shape / 2

Describe alternatives you've considered
We could change the documentation instead but storing the centroid seems more coherent with the rest of the tools we deal with, and is likely less jittery than the corners.

Additional context
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