Example gallery suggestion: spectrum.average_photon_energy

[RDaxini]

I am wondering what level of interest there is for an example gallery addition demonstrating the use of spectrum.average_photon_energy.

I think an example calculation of the APE of several spectra could help users understand how to interpret the parameter (high values => blue-shifted spectrum, lower values => red-shifted spectrum) as well as guide users on how to use the function of course.

Since we have the spectrum.spctrl2 function to simulate spectra, but the output is not usable directly as an input into spectrum.average_photon_energy, I think some example code linking the two would also be helpful so that users have a guide on how to simulate and characterise spectral distributions all within pvlib-python.

I'd be happy to create this example if the community thinks it'd be useful.

[AdamRJensen]

That sounds great to me!

[RDaxini]

I'll start working on this soon then and create a PR once I have something ready 👍