Agrivoltaics - PAR diffuse fraction model

[echedey-ls]

• Closes Photosynthetically active radiation decomposition model #2047
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Adds the model described in #2047.

Current implementation and next steps

Added a new module called par. I still haven't added the index and API entries since I would like to have the green light before pursuing this path.

I'm a bit hesitant about the tests here. I haven't found any straightforward way to make the tests from the papers. They check the mathematical integrity of the implementation, if that's a thing.

Docs links

• API Index
• Model
• Gallery example

[AdamRJensen]

Just an early stage review.

[AdamRJensen]

@kandersolar Do we have a standard format for specifying units? I've seen at least three different methods in our documentation:

• ... active radiation in W/m^2.
• ... active radiation. W/m^2.
• ... active radiation. [W/m^2]

[kandersolar]

This question just came up elsewhere as well, with an additional contender of :math:`W/m^2`.

I don't think we have anything that rises to the level of a "standard", but I personally favor [W/m^2] unless there is good reason for something else. Reasons:

• It makes sense to me for the unit to be visually separated from the text description (i.e. not in the same sentence) as it may prevent ambiguity when other quantities are mentioned in the description. So "... radiation in W/m^2" is out.
• The :math: version results in bold font that looks a little jarring to me at the end of each line.
• I think (but am not 100% confident) that [W/m^2] is the most common form currently in pvlib, so consistency supports keeping it.

If I am being honest, the main reason for me might just be that I am used to [W/m^2]. Anyway, that's my two (three?) cents!

[echedey-ls]

I agree with the reasoning. There's still another alternative, which is using the ascii character superscript two:
[W/m²]
I personally prefer this one, to me it's like the caret adds visual noise.