Add a function for integrating SED flux, for different types of radiative processes

[grzegorzbor]

The SedFluxIntegrable is a mixin that can be added to all classes that implement sed_flux function.
It provides 2 functions, one for calculating energy flux integral, and the other for photon flux integral, over frequency range.

[cosimoNigro]

Hello @grzegorzbor, many thanks for this first contribution!

What about making this PR a bit more general?
That is, instead of SedFluxIntegrable, I think it would be much better to directly introduce a general SedFlux (or Flux) class (from which all the radiative processes would inherit) that not only performs the integration of the flux, but also handles all these conversions that we deal with (and keep repeating) in several parts of the package. For example, the class would return the flux either as $F_{\nu} [{\rm erg} {\rm cm}^{-2} {\rm s}^{-1} {\rm Hz}^{-1}]$, as $\nu F_{\nu} [{\rm erg} {\rm cm}^{-2} {\rm s}^{-1}]$ or as a differential flux in energy, ${\rm d} \phi / {\rm d} E [{\rm eV}^{-1} {\rm cm}^{-2} {\rm s}^{-1}]$ - what we actually pass to the Gammapy wrapper and what you would finally like to integrate to obtain the integral flux in ${\rm cm}^{-2} {\rm s}^{-1}$.

[cosimoNigro]

Hi @grzegorzbor, I took the liberty of doing some fixes and making the code a bit slimmer.
Let me know if you like the latest changes.
Thank you!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e6f5783) 96.95% compared to head (be47b3f) 97.02%.

Additional details and impacted files

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##           master     #153      +/-   ##
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+ Coverage   96.95%   97.02%   +0.06%     
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  Files          40       43       +3     
  Lines        3319     3395      +76     
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+ Hits         3218     3294      +76     
  Misses        101      101              

Flag	Coverage Δ
unittests	97.02% <100.00%> (+0.06%)	⬆️

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