Create calculate_avg_photon_energy function

[RDaxini]

I think a function to calculate the average photon energy (APE) using spectral irradiance data as an input would be a useful feature in pvlib.

Additional context
This would support PR #2126. While there are some discussions ongoing regarding that PR in Issue #2065, I think this may still be a useful feature in pvlib anyway regardless of the outcome of that PR.

The function would be in pvlib/spectrum/spectral_irradiance.py following the resolution of #2125 (I will create a PR for that soon issue soon)

I am wondering what people think about naming this function. Possibilities:

calculate_avg_photon_energy
calculate_average_photon_energy
calculate_ape
Switch "calculate" for "get" or "calc", or just omit this first word entirely?
Alternatives?

Any other thoughts on this?

[echedey-ls]

or just omit this first word entirely?

+1 for average_photon_energy

[AdamRJensen]

or just omit this first word entirely?

+1 for average_photon_energy

+2