Proofreading improvements

[lorenzocerrone]

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

Unrelated changes:

• [x] Imrpove defaults in import task
• [x] Fix cropping for 2D
• [x] Handle gracefully edge case when the rectangle is not drawn in XY plane
• [x] Fix #351

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

[codecov[bot]]

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 :warning:
plantseg/core/image.py	34.09%	29 Missing :warning:
plantseg/tasks/dataprocessing_tasks.py	17.64%	14 Missing :warning:
plantseg/viewer_napari/widgets/io.py	38.09%	13 Missing :warning:
plantseg/viewer_napari/widgets/prediction.py	33.33%	12 Missing :warning:
plantseg/viewer_napari/widgets/dataprocessing.py	14.28%	6 Missing :warning:
plantseg/viewer_napari/widgets/segmentation.py	0.00%	1 Missing :warning:
plantseg/viewer_napari/widgets/utils.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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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