Release 4.0 bugfix

[HaleySchuhl]

Describe your changes
dual threshold bugfix propogated down

Type of update
Is this a:

• Bug fix
• New feature or feature enhancement
• Update to documentation
• Work in progress

Associated issues
Reference associated issue numbers. Does this pull request close any issues?

Additional context
Add any other context about the problem here.

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 #1146 (8e1200f) into release-4.0 (f5da302) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1146   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              164       164           
  Lines             7007      7010    +3     
=============================================
+ Hits              7007      7010    +3     

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)