Implement ANTS-2D bifacial irradiance model

[kandersolar]

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@AdamRJensen, @cwhanse, and I have a paper describing a new bifacial irradiance model called ANTS-2D. It is similar to pvlib's infinite_sheds model, but extended to allow:

• a discretized module surface (for, e.g., cell-level irradiance values)
• a discretized ground surface, with variable albedo
• sloped terrain, in the fashion of pvlib's existing functionality in that area
• the Perez transposition model, in addition to Hay-Davies and isotropic
• computation of ground-level irradiance (for agriPV)
• fast computation by using analytical integrated view factors instead of burdensome numerical integrals

Details available open-access here: https://doi.org/10.1109/JPHOTOV.2026.3677506

This PR is rather large. To summarize:

• Add g0 and g1 parameters to the view factor functions in pvlib.bifacial.utils. These are analogous to x0 and x1 in vf_row_sky_2d_integ and extend the functions to subset the ground surface.
• Change vf_ground_sky_2d_integ to use Hottel's crossed-string rule instead of burdensome numerical integration. This makes the npoints and vectorize parameters unnecessary.
• Change some signatures in pvlib.bifacial.utils to be cleaner with the new calculations.
• Minor edits in pvlib.bifacial.infinite_sheds to accommodate the utils changes
• Create pvlib.bifacial.ant2d, which houses the model itself and uses the new utils functionality.

Let me know if it would help reviewers to split it up and review separate PRs, starting with utils.

[cwhanse]

Review of file ants2d.py. OK with file infinite_sheds.py.

[cwhanse]

Thorough tests!

[cwhanse]

Very minor nit: "sun behind the modules" I think I would comment this case as "module facing away from the sun" since the function being tested returns "fraction...shaded from direct irradiance by the row in front toward the sun." It is implied that _shaded_fraction is specific to one face of a module, which (in this case) is face pointed away from the sun.