Add image pair operation widget/task/functionals/docs

[qin-yu]

This is a missing widget from v1 and needed by a neighbour lab.

[codecov]

Codecov Report

Attention: Patch coverage is 92.07921% with 8 lines in your changes missing coverage. Please review.

Project coverage is 65.25%. Comparing base (5ffaf72) to head (a782a70).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
plantseg/viewer_napari/widgets/dataprocessing.py	33.33%	4 Missing ⚠️
plantseg/tasks/dataprocessing_tasks.py	40.00%	3 Missing ⚠️
...ests/functionals/dataprocessing/test_image_math.py	98.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #370      +/-   ##
==========================================
+ Coverage   64.71%   65.25%   +0.53%     
==========================================
  Files          71       72       +1     
  Lines        5201     5301     +100     
==========================================
+ Hits         3366     3459      +93     
- Misses       1835     1842       +7     

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