For comparison purposes, I'm running a lot of models using the ModelChain object.

I'd like to vary the Perez parameters to see if my choice of parameters is having a significant effect, but using the ModelChain interface does not support changing this argument, which is passed to system.get_irradiance in ModelChain.prepare_inputs.

This makes it so a ModelChain always uses the default allsitescomposite1990 coefficients when using perez.

We can work around this by using ModelChain.run_model_from_poa rather then ModelChain.run_model, but this seems rather clunky if all you want to change is the Perez coefficients.

I'll submit a PR with a fix if there's interest. Thoughts?