Update PSII dataset names

[nfahlgren]

Describe your changes
Renames PSII_data dataset attributes from darkadapted and lightadapted to ojip_dark and ojip_light, based on the name of the method, OJIP. This will allow future addition of PAM measurements.

Type of update
Is this a: update

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 #1309 (3636334) into release-4.0 (6f2f7d4) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1309   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              157       157           
  Lines             6757      6759    +2     
=============================================
+ Hits              6757      6759    +2     

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/npq.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/yii.py	100.00% <100.00%> (ø)
plantcv/plantcv/classes.py	100.00% <100.00%> (ø)
...lantcv/plantcv/photosynthesis/read_cropreporter.py	100.00% <100.00%> (ø)
...cv/plantcv/photosynthesis/reassign_frame_labels.py	100.00% <100.00%> (ø)
...ntcv/plantcv/visualize/chlorophyll_fluorescence.py	100.00% <100.00%> (ø)

[HaleySchuhl]

I'm not sure I've got data to test on for OJIP.. I don't think the sample data I have for Katie is good to test this. could you please point me to a test image? @nfahlgren