Simplify/DRY some infinite_sheds/bifacial.utils calculations

[kandersolar]

• [ ] Closes #xxxx
• 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.

Projected solar zenith angle (tan_phi) is calculated several times in the infinite_sheds workflow. This PR calculates it once and passes it around, simplifying the relevant utility functions and their signatures.

For context, this is a step towards future enhancements I intend to make to pvlib.bifacial, which would benefit from working with tan_phi instead of the base solar position and module angles.

[echedey-ls]

You've probably got the email, but docs are failing with:

This failing example

Extension error:
Here is a summary of the problems encountered when running the examples:
Unexpected failing examples (1):
    ../../examples/agrivoltaics/plot_agrivoltaics_ground_irradiance.py failed leaving traceback:
    Traceback (most recent call last):
      File "/home/docs/checkouts/readthedocs.org/user_builds/pvlib-python/checkouts/2559/docs/examples/agrivoltaics/plot_agrivoltaics_ground_irradiance.py", line 145, in <module>
        unshaded_ground_fraction = pvlib.bifacial.utils._unshaded_ground_fraction(
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    TypeError: _unshaded_ground_fraction() got an unexpected keyword argument 'surface_azimuth'

My opinion is that it's bad practice to use internals in public-facing examples. But a fix, using internals or not, is okay with me.

Also, I think this deserves a whatsnew 👀
Nice improvement.

[kandersolar]

My opinion is that it's bad practice to use internals in public-facing examples.

Agreed. We should consider making that ground shaded fraction function public.