Define type for markers in watershed

[nfahlgren]

Describe your changes
Scikit-image stopped converting the input markers to watershed to int32 by default and we do not explicitly define the data type of markers in pcv.segment_image_series or pcv.morphology.fill_segments. Tests started failing with scikit-image v0.21.

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 #1175 (d39fe3b) into 4.x (250711e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               4.x     #1175   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          161       161           
  Lines         6890      6890           
=========================================
  Hits          6890      6890           

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/morphology/fill_segments.py	100.00% <100.00%> (ø)
plantcv/plantcv/segment_image_series.py	100.00% <100.00%> (ø)