Add Batzelis 2017 simple nonlinear PV model

[kandersolar]

• Closes Datasheet-only nonlinear model for Pmp/Imp/Vmp/Isc/Voc #2562
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• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
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[kandersolar]

This PR currently adds three new functions. As they are related to each other I thought it would be helpful to review them together, but I can split the PR into smaller pieces to make it more digestible if needed.

To summarize the three functions:

• pvlib.ivtools.sdm.fit_desoto_batzelis: estimate De Soto SDM parameters from datasheet values
• pvlib.singlediode.batzelis_keypoints: estimate Pmp/Imp/Vmp/Isc/Voc from SDE parameters
• pvlib.pvarray.batzelis: estimate Pmp/Imp/Vmp/Isc/Voc from datasheet values

The third, effectively, combines the first two with calcparams_desoto, but with some algebraic manipulations to make the computation more compact. I think there is also a minor difference in the calcparams_desoto calculations. All three come from the paper linked in #2562.

Questions for reviewers:

• Currently alpha_sc has units [1/K] in some places and [A/K] in others. Better to diverge from the reference (which uses [1/K]) and use [A/K] everywhere for consistency with the rest of pvlib?
• Parameter names likely also should be changed for consistency. i_sc instead of isc0, etc?
• Are the current modules the best places for these new functions?

[cwhanse]

Some reactions.

[cwhanse]

I would prefer alpha_sc and beta_voc to be required in units of A/K and V/K, respectively. The parameters alpha_sc and beta_voc in sdm.fit_desoto, sdm.fit_cec_sam and other function (notably, pvsystem.calcparams_desoto) use those units.

Do we want the STC parameters to have the same names as in pvsystem.sapm? I'm not a fan of the SAPM names, which derive from some old spreadsheet's column headings. But they are the same quantities.

[kandersolar]

units of A/K and V/K

done

Do we want the STC parameters to have the same names as in pvsystem.sapm? I'm not a fan of the SAPM names,

Me neither. fit_desoto uses just the usual v_mp etc. This PR now uses v_mp etc. too, although those are obviously not ideal either.

[cwhanse]

A question about the method: does batzelis produce the same results as fit_desoto_batzelis + batzelis_keypoints? It is not clear to me that should be the case.

[kandersolar]

A question about the method: does batzelis produce the same results as fit_desoto_batzelis + batzelis_keypoints? It is not clear to me that should be the case.

Close but not exactly the same, in my testing. The latter has to be fit_desoto_batzelis + calcparams_desoto + batzelis_keypoints, and I think there is at least a difference in the calcparams_desoto step (regarding dEgdT). I think there are differences elsewhere too, but I can't point to any specifics.