Improvements automatic instance segmentation

[constantinpape]

Follow up tasks to #305

• Load the decoder from published finetuned models, pending Bioimageio Model Creation #227 to bring models to the zoo.
• Expose more parameters for instance segmentation from decoder, revisit default value for smoothing (we still see some artifacts).
• Implement 3d post-processing to merge across small missing gaps in z.
• Improve AMG for overlapping objects, see Artifactual Segmentation "Rings" after Automatic Segmentation (2D) #416

[constantinpape]

Fixed by #462.