calculate energy yield - integrate with pvlib-python or other changing spectra models

[jgeisz]

proxy spectra:
https://github.com/Ripalda/Tandems
J. M. Ripalda, J. Buencuerpo and I. Garcia, “Solar cell designs by maximizing energy production based on machine learning clustering of spectral variations.” Nature Communications 9, p. 5126 (2018).