Set num_labels to num ROIs

[nfahlgren]

Describe your changes
pcv.create_labels was incorrectly setting the number of labels to the number of unique labels in the labeled mask. When plants are missing and ROIs are supplied, this can under count the number of labels. Instead we changed the number of labels to the number of input ROIs, when relevant.

Type of update
Is this a: Bug fix

For the reviewer
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• PR functionality reviewed in a Jupyter Notebook
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• Documentation tested
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[codecov]

Codecov Report

Merging #1340 (56d6cc8) into release-4.0 (643c68b) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1340   +/-   ##
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  Coverage       100.00%   100.00%           
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  Files              156       156           
  Lines             6857      6857           
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  Hits              6857      6857           

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

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

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