cpr-chas

@mikofski, @kanderso-nrel, we are looking at adding some self-shading to our fork and we'd love to do this in a way that allows us to make a useful contribution to...

> > > @cpr-chas I think `pvlib.shading` should contain functions for: > > * effects-level shading models (e.g., [Deline's ](https://www.researchgate.net/publication/273227488_A_simplified_model_of_uniform_shading_in_large_photovoltaic_arrays)approach) > > * functions that adjust direct and diffuse irradiance...