[BUGFIX] Inconsistent naming of cross_axis_tilt / cross_axis_slope

[echedey-ls]

• Closes cross_axis_slope vs cross_axis_tilt #2334
• I am familiar with the contributing guidelines
• Tests added
• [NA] 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`).
• [NA] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
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pvlib.shading.shaded_fraction1d uses 'cross_axis_slope' as an input, while pvlib.tracking.singleaxis uses 'cross_axis_tilt'. I think these are the same thing - is that correct? If so, would it make sense to update one of them to be consistent?

Extract from the body message, OP @williamhobbs, #2334 .

Also adds variable description to nomenclature page.

Relevant doc pages

• https://pvlib-python--2543.org.readthedocs.build/en/2543/user_guide/extras/nomenclature.html#term-cross_axis_tilt
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.shading.shaded_fraction1d.html
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.tracking.singleaxis.html

[echedey-ls]

Currently targets v0.13.1, but I leave this decision up to anybody else (please, assign milestone accordingly). I will update deprecations tracker when merged.

[AdamRJensen]

Suggested change

removal="0.15.0",

We usually don't specify a removal version as there's not standard timeline for when minor versions come out. If we really wanted to specify a removal, I think this should be in the form of a date, e.g., removal="Earliest September 2026".

[echedey-ls]

I never thought of that, but looks a promising idea

[AdamRJensen]

I'm interested in what @wholmgren thinks of this?

[AdamRJensen]

The original thread #2334 contained multiple views in favor of cross_axis_slope over cross_axis_tilt. This PR currently implements the latter. Is there a reason for this?

[RDaxini]

I guess we should go for this syntax given the current style guide and the rest of the nomenclature page

Suggested change

	Cross-axis tilt angle [°].
	Cross-axis tilt angle. [°]

[RDaxini]

Just a few style suggestions, mostly my personal preference. Feel free to take them all, cherry pick a few, or reject them all.
Removed "by", swapped "degrees" for °, added a comma after east.

Suggested change

	Cross-axis tilt is measured by using a right-handed convention.
	For example, trackers with axis azimuth of 180 degrees (heading south)
	will have a negative cross-axis tilt if the tracker axes plane slopes
	down to the east and positive cross-axis tilt if the tracker axes plane
	slopes up to the east.
	Use :func:`~pvlib.tracking.calc_cross_axis_tilt` to calculate
	``cross_axis_tilt``
	Cross-axis tilt is measured using a right-handed convention.
	For example, trackers with axis azimuth of 180° (heading south)
	will have a negative cross-axis tilt if the tracker axes plane slopes
	down to the east, and positive cross-axis tilt if the tracker axes plane
	slopes up to the east.
	Use :func:`~pvlib.tracking.calc_cross_axis_tilt` to calculate
	``cross_axis_tilt``

[echedey-ls]

Thanks for the reviews.

@AdamRJensen , yeah, my fault. This is a stale branch I had locally - I guess I took that decision to conform with what already was the term prior to the newer feature and just skimmed through the discussion. I will change this whenever I can to reflect what was discussed.