Add sample label param

[nfahlgren]

Describe your changes
Adds a global parameter to Params to set the sample label. The global parameter is the default value for the label parameter for all functions that store data in Outputs. Code is added to each analysis function to set label from params when label=None, but allows label to be overridden by user input. Does not break existing functionality.

Type of update
Is this a: New feature or feature enhancement

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

[nfahlgren]

To do:

• within_frame
• homology functions
• morphology functions
• updating.md

[codecov]

Codecov Report

Merging #1328 (2749a0c) into release-4.0 (151e4ba) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1328   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              156       156           
  Lines             6772      6829   +57     
=============================================
+ Hits              6772      6829   +57     

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/analyze/bound_horizontal.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/bound_vertical.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/color.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/grayscale.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/npq.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/size.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/spectral_index.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/spectral_reflectance.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/thermal.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/yii.py	100.00% <100.00%> (ø)
... and 19 more