Improvements for Cellpose segmentation

[pakiessling]

Made the following changes:

• Cellpose weights are now correctly used, previously Cyto3 was always applied instead
• Added an optional parameter clahe_kernel_size for skimage.exposure.equalize_adapthist which can be relevant for very big cells or patches. The default still is 1/8 of patch width and height
• Blur and CLAHE can be disabled by setting the parameter to 0
• If segmentation is run on multiple channels (e.g Nucleus marker + cell boundary marker) blur and CLAHE are now run on every channel separately which prevents spillover of intensities

[quentinblampey]

Thanks @pakiessling, looking very good overall, I added a few comments!

[quentinblampey]

All the tests passed, I'm merging, thanks again @pakiessling 🎉