remove max_val param from threshold.gaussian()

[afinit]

Describe your changes

• Removes max_val param from the threshold.gaussian method
• updates function documentation
• updates updating.md
• fixes tests broken by change

Type of update
feature enhancement

Associated issues
completes one task from this ticket: #1194

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 #1214 (0befc9c) into release-4.0 (94cd510) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1214   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              163       163           
  Lines             6994      6994           
=============================================
  Hits              6994      6994           

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

Flags with carried forward coverage won't be shown. Click here to find out more.

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