Consider renaming x to y in bifacial functions

[cwhanse]

The reference and local coordinate systems for bifacial functions align x to the East (reference) or along a tracker axis (local), and y to the North (reference) or perpendicular to the tracker axis and parallel to the ground. z is always Up.

Some bifacial functions, e.g., https://github.com/pvlib/pvlib-python/blob/6553710e8e87f0931846eba2abc6f21c19ebdadf/pvlib/bifacial/utils.py#L95, require a parameter x defined as "Position on the ground between two rows", or "Position on the row's slant length". In the coordinate systems, x corresponds to a distance in the y direction.

[adriesse]

a parameter x defined as "Position on the ground between two rows", or "Position on the row's slant length". In the coordinate systems, x corresponds to a distance in the y direction.

Those description sound like the position may not be measured along any of those three axes though...

The letter x does get used and abused rather a lot, poor thing!