Proofreading improvements

[lorenzocerrone]

• General cleanup of proofreading code. Remove redundant copying of the data in the proofreading handler.
• Add methods to save and construct a plantseg image from h5.
• Save proofreading state to disk as h5
• Proofreading seems to work now on all orientations (to be tested more extensively)

Unrelated changes:

• Imrpove defaults in import task
• Fix cropping for 2D
• Handle gracefully edge case when the rectangle is not drawn in XY plane
• Fix Areas of GUI improvement after v2.0.0a4 #351

@qin-yu test it a bit and let me know if the behavior did not change

[codecov]

Codecov Report

Attention: Patch coverage is 29.18919% with 262 lines in your changes missing coverage. Please review.

Project coverage is 64.81%. Comparing base (d19176e) to head (f8bfe4b).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
plantseg/viewer_napari/widgets/proofreading.py	27.62%	186 Missing ⚠️
plantseg/core/image.py	34.09%	29 Missing ⚠️
plantseg/tasks/dataprocessing_tasks.py	17.64%	14 Missing ⚠️
plantseg/viewer_napari/widgets/io.py	38.09%	13 Missing ⚠️
plantseg/viewer_napari/widgets/prediction.py	33.33%	12 Missing ⚠️
plantseg/viewer_napari/widgets/dataprocessing.py	14.28%	6 Missing ⚠️
plantseg/viewer_napari/widgets/segmentation.py	0.00%	1 Missing ⚠️
plantseg/viewer_napari/widgets/utils.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #355      +/-   ##
==========================================
- Coverage   66.33%   64.81%   -1.52%     
==========================================
  Files          70       70              
  Lines        4987     5159     +172     
==========================================
+ Hits         3308     3344      +36     
- Misses       1679     1815     +136     

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