Cellpose relabeling & masked_loading interaction leads to relabeling objects multiple times

[jluethi]

There is an issue with the relabeling in the cellpose segmentation task happening after the masked loading wrapper. In that way, it relabels objects of other masks (that were in the background when processing a given object) again.

This leads to split segmentations and inefficient relabeling.

I will come up with a test case to show this and implement a bugfix to fix this.