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]

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #153 +/- ## ========================================== + Coverage 96.95% 97.02% +0.06% ========================================== Files 40 43 +3 Lines 3319 3395 +76 ========================================== + Hits 3218 3294 +76 Misses 101 101 ``` | [Flag](https://app.codecov.io/gh/cosimoNigro/agnpy/pull/153/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Cosimo+Nigro) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/cosimoNigro/agnpy/pull/153/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Cosimo+Nigro) | `97.02% <100.00%> (+0.06%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Cosimo+Nigro#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.