Functions to fit and convert IAM models

[cwhanse]

It would be useful to be able to convert parameters between IAM models (e.g., physical to martin_ruiz). To convert from an interpolation approach to a model described by an equation, it would also be useful to have a "fit" function.

Describe the solution you'd like

Add fit and convert functions to pvlib.iam.py. convert could be implemented as

target_params = convert(source_model, source_params, target_model)

Add an example to the gallery showing how to use these functions, and demonstrating that a converted model yields similar effective insolation for a representative system.

Describe alternatives you've considered
An example showing how to do this. The example would contain the fit/convert code, which seems like a less-than-useful implementation.