Add converters between single diode models

[cwhanse]

Is your feature request related to a problem? Please describe.
pvlib provides three single diode models: CEC, PVSyst and De Soto, with the first two being of more interest. pvlib copies and distributes the SAM file containing parameters for the CEC model.

It seems reasonable that some would like to convert from CEC parameters to a set of PVSyst model parameters, such that the conversion produces equivalent power and energy simulations. Something like pvlib.ivtools.sdm.convert modeled after pvlib.iam.convert.

Interested in reactions to this idea:

• Would you use it?
• What criteria should be used to judge that two diode models are "equivalent?"

[mikofski]

Yes, I would use it