Add parameters to mean adaptive thresh

[JorgeGtz]

Describe your changes
Adding parameters of block size and offset to both, mean and Gaussian adaptive threshold functions.

This will add flexibility to tune the behavior of the adaptive threshold depending on the image (resolution, size of the objects, and degree of change in brightness).

We have been using these functions as automatic methods, with block size and offset fixed, but that is not an optimal usage of the adaptive thresholds. The important characteristic of these adaptive methods is that they use a different threshold value for each pixel, depending on the value of the pixels in a surrounding block.

Type of update
Is this a:

• New feature or feature enhancement

Associated issues

Additional context

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[codecov]

Codecov Report

Merging #937 (8ce5f85) into release-4.0 (4390066) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 8ce5f85 differs from pull request most recent head 00c5d74. Consider uploading reports for the commit 00c5d74 to get more accurate results

@@              Coverage Diff               @@
##           release-4.0      #937    +/-   ##
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  Coverage       100.00%   100.00%            
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  Files              161       147    -14     
  Lines             6840      6640   -200     
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- Hits              6840      6640   -200     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
plantcv/parallel/multiprocess.py	100.00% <100.00%> (ø)
plantcv/plantcv/find_objects.py	100.00% <100.00%> (ø)
plantcv/plantcv/hyperspectral/analyze_spectral.py	100.00% <100.00%> (ø)
plantcv/plantcv/roi/roi2mask.py	100.00% <100.00%> (ø)
plantcv/plantcv/roi/roi_methods.py	100.00% <100.00%> (ø)
plantcv/plantcv/threshold/threshold_methods.py	100.00% <100.00%> (ø)
plantcv/plantcv/transform/color_correction.py	100.00% <100.00%> (ø)
...lantcv/plantcv/visualize/auto_threshold_methods.py	100.00% <100.00%> (ø)
plantcv/plantcv/transform/__init__.py
plantcv/learn/__init__.py
... and 12 more