Fix threshold dual channels returning bad dtype

[JorgeGtz]

Describe your changes
function threshold.dual_channels was returning a binary image of dtype depending on the type of max_value. By default and most of the time it returned a 64 bits image which is incompatible with the rest of PlantCV functions.

Type of update
Is this a:

• Bug fix

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 #1144 (0036dca) into 4.x (f0be5e0) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               4.x     #1144   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          161       161           
  Lines         6849      6852    +3     
=========================================
+ Hits          6849      6852    +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)