Refactor variable names for clarity in APE example

[AdamRJensen]

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I used the APE gallery example for teaching, and students found it confusing that the variable names were inconsistent with the inputs, so I suggest changing it.

[kandersolar]

Those names were copied from plot_spectrl2_fig51A.py. I was going to suggest that we change them there too, but I think it makes more sense to keep them there since it is tied to the SPECTRL2 report, whereas this example doesn't have that context.

[echedey-ls]

Those names were copied from plot_spectrl2_fig51A.py. I was going to suggest that we change them there too, but I think it makes more sense to keep them there since it is tied to the SPECTRL2 report, whereas this example doesn't have that context.

I agree with you. You can also put AKA comments or link to :term:'s, if some consistency with pvlib's nomenclature is deemed important enough.