pvsystem.Array.get_irradiance raises an error if solar_zenith is passed as a float

Bug Description pvsystem.get_irradiance documentation says solar_zenith can be passed as a float or a Series. However, if dni_extra is not defined and solar_zenith is passed as float, then solar_zenith.index is not defined and the call to irradiance.get_extra_irradiance with solar_zenith.index as an argument fails.

To Reproduce Steps to reproduce the behavior: test_sys = pvsystem.PVSystem() test_sys.get_irradiance(0.0, 0.0, 0.0, 0.0, 0.0)

Error message AttributeError: 'float' object has no attribute 'index'

Expected behavior Return a DataFrame containing the irradiance components, as specified in the documentation.

Possible solutions

1. Ask for datetime if dni_extra is not passed as argument
2. Remove float from supported types for solar_zenith

Versions:

• pvlib.__version__: 0.9.0
• pandas.__version__: 1.3.3
• python: 3.9

[wholmgren]

*must be a Series with a DatetimeIndex.

I also like @kanderso-nrel’s suggestion in https://github.com/pvlib/pvlib-python/issues/1338#issuecomment-967232590

[adriesse]

Most or all of the bottom layer functions work with scalars and I think that’s important. May be less so for the classes, but still desirable.