Implement possibility to refine masks after CellPose segmentation

[MaximLippeveld]

CellPose produces a mask for one channel only. Running CellPose for every channel is too computationally intensive. This can be solved by using CellPose to get a mask that covers the entire cell, and subsequently refining that mask per channel.

[MaximLippeveld]

Refining the mask within the parent patch proved difficult, because very little context is available around the cell. It should be feasible though, so I will revisit this in the future.

Currently it is only possible to generate a new CellPose mask for a second channel. These segment are then assigned to the parent cell based on overlap.