Change g_poa_effective to effective_irradiance

[AdamRJensen]

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g_poa_effective is an alternative name for effective_irradiance. The latter is the standard in pvlib (at least the most widely used).

This is a breaking change, but I don't see how this can be deprecated in a nice way.

The previous definition was also not correct. IMO it doesn't you can't use the term "broadband" about effective irradiance which has potentially been modified for spectral effects.

g_poa_effective;broadband plane of array effective irradiance.