All-sky adjustment to clear sky spectrum

[mikofski]

Is your feature request related to a problem? Please describe.
Recent work demonstrated spectral affected by cloud, clear sky under predicts shift
https://www.pvlighthouse.com.au/cms/simulation-programs/sun-calculator

Describe the solution you'd like
Implement Ernst correlation
Ernst developed simple correction to adjust clearly spectrum to cloudy.

Describe alternatives you've considered
Use farms nit

Additional context
https://github.com/ISFH/SunCalculatorLibrary
https://doi.org/10.1016/j.solmat.2016.08.008

[RDaxini]

Related: #1669 (?)

[kandersolar]

Check out #1201, I think it's implementing the Ernst method (although stale for 4 years now).

[RDaxini]

Check out #1201, I think it's implementing the Ernst method (although stale for 4 years now).

I'll try to have a look over the PR some time this week. If the original contributor is no longer interested, I could take over the work. I think having some kind of cloud correction to clear sky spectral simulations in pvlib would be a valuable addition.