Add analyze.spectral_index function

[nfahlgren]

Describe your changes
Adds the function spectral_index to the analyze subpackage. The function is a modified version of pcv.hyperspectral.analyze_index that utilizes the iterative analysis framework with labeled masks and produces an altair plot of recorded values.

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

[codecov]

Codecov Report

Merging #1179 (253436f) into release-4.0 (edd805f) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1179   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              173       174    +1     
  Lines             7684      7727   +43     
=============================================
+ Hits              7684      7727   +43     

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/analyze/__init__.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/spectral_index.py	100.00% <100.00%> (ø)