Add wavelength range arguments to spectrum.average_photon_energy

[RDaxini]

Is your feature request related to a problem? Please describe.
The calculated APE value depends on the integration (wavelength) limits. Currently, spectrum.average_photon_energy performs the calculation for the full wavelength range of the input. The user is unable to calculate the APE based on a limited section of the spectrum.

Describe the solution you'd like
Add two new arguments to the function: upper_wavelength and lower_wavelength
Argument names are open for discussion
Default values: None, if value is None then the APE is calculated based on the full wavelength range of the input. Are there any alternative suggestions for this behaviour?

Additional context
I will open a PR soon for this, after wrapping up a couple of other open PRs first.

[adriesse]

It's pretty easy to limit the wavelength range already by adjusting the input, isn't it?

[RDaxini]

@adriesse I agree it is, but I thought it might be a more nicely packed 'complete package' if this functionality was integrated into this function? Just an idea though, I don't have to go ahead with it if it seems unnecessary:)