Return shaded fraction values from infinite_sheds.get_irradiance and get_irradiance_poa

[williamhobbs]

Is your feature request related to a problem? Please describe.
Shaded fraction is useful for estimating shade losses. It is already calculated internally when using infinite_sheds.get_irradiance and infinite_sheds.get_irradiance_poa. It would be nice if values for shaded fraction were returned.

Describe the solution you'd like
Add shaded fraction to the returned output dataframe.

Describe alternatives you've considered
Calculating shaded fraction some other way.

Additional context
See discussion here: #1869. A separate general-purpose public function is in the works elsewhere.

[kandersolar]

Closed by #1871