Major refactor of all PlantSeg process and add support for napari as viewer

[lorenzocerrone]

• [x] Unet predictions follow the AbstractStep like the rest of the pipeline
• [x] New in-memory array loader and predictor
• [x] Implement all steps as pure functions (preprocessing, predictions, segmentation)
• [x] Unified all file io in a new subpackage plantseg.io
• [x] Fix testing
• [x] Add napari viewer and widgets
• [x] Add tracking of napari workflow for headless predictions