Document flux functions

[ErwanQuintin]

Document the model.abc.photon_flux and model.abc.energy_flux functions, showing how to use them (and how to apply parameters to the loaded SpectralModel object)

[renecotyfanboy]

I'll also add wrappers so that you can enter floats both as parameters and energy limits. Add better integration too

• Document the photon flux and energy flux
• Integrate between more than 2 energies
• Ensure the functions can admit float arguments
• Add unit tests

[ErwanQuintin]

Emphasize the fact that it's a single trapezoid integration, and using it to compute fluxes in a large band requires to sum over several smaller bins