add pvwatts_dc_marion

[williamhobbs]

• Closes Nonlinear adjustment to pvwattsv5 dc model #2566
• 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.

Still has some work to do, but I thought I would share what I've started to get feedback. Tagging @kandersolar, since we briefly discussed this.

[cwhanse]

My reaction: I think adding k as an optional input to the existing pvwatts_dc would be better. The base equation (when k is 0) is the same. Adding k as optional with default 0 sidesteps the question about a default k value.

[williamhobbs]

@cwhanse, that sounds good to me.

Do you suggest I close this PR and open a new one?

And what would you think about adding an optional Boolean input parameter that lets you “cap” the Marion correction at 1000 W/m^2? It would default to False (following Marion 2008 if you pass k!=0), but if you set it True, it sets output equal to regular pvwatts above 1000. This prevents efficiency from climbing above STC at POA above 1000.

[cwhanse]

Up to you if you want to close this PR and start fresh.

The cap sounds like a good addition for the pvlib implementation.

[williamhobbs]

Ok, I'll close this PR and start a new one.

Edit: see new PR, #2569.