The points CSV could be optional

[vreuter]

The CSVs in _nuclear_masks_visualisation could be optional, as they're only used to fix a predetermined label/number to each nucleus, and to designate the centroid to position that label at the center of the nucleus. This is good for looptrace but could be made more general, so that the value of the label is taken directly from the nonzero integer that defines each region, and the centroids could be computed from the regions themselves as well.