Rename to ksize in adaptive thresholding methods

[HaleySchuhl]

Describe your changes
For consistency, call the parameter ksize instead of block_size since we use ksize for kernel size elsewhere.

Type of update
Is this a:

• Update

Associated issues
#1304

Additional context
Add any other context about the problem here.

For the reviewer
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• 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
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[codecov]

Codecov Report

Merging #1325 (7ce560a) into release-4.0 (74de53b) will not change coverage.
Report is 1 commits behind head on release-4.0.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1325   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              157       157           
  Lines             6757      6757           
=============================================
  Hits              6757      6757           

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

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Files Changed	Coverage Δ
plantcv/plantcv/threshold/threshold_methods.py	100.00% <100.00%> (ø)