• 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.

This PR is part of my GSoC 2026 project. One of the goals is to add support for return_components to all diffuse transposition models in pvlib, which currently is supported by some models (e.g. perez) but not others. This parameter allows the user to choose whether they want only the total diffuse irradiance, or are interested in the split between different diffuse components (sky diffuse, circumsolar, horizon brightening).

This first PR starts by implementing return_components in reindl.

Reindl expands the Hay and Davies model, which considers only the isotropic and circumsolar components, and is defined as:

with A Rb being the circumsolar component and the rest being the isotropic component.

Reindl expands this by adding a horizon brightening component, but this new component is not additive but rather multiplied by the existing isotropic component:

In my implementation, I essentially considered that the isotropic term was:

while the horizon brightening term was:

I updated the function docstrings accordingly. I haven't yet added a test. I saw that e.g. haydavies has two separate tests, for return_components=False and return_components=True. However, from what I gathered, the test values came from the matlab implementation of pvlib, and therefore it seems kind of silly to create a test using values derived from the python implementation to test the python implementation. Curious as to what are the community's thoughts on this.