Crosscheck of External Compton radiation

[cosimoNigro]

We should try to crosscheck with different approaches the External Compton radiation:

• crosscheck against the literature;
• crosscheck against another code;
• crosscheck against back-of-the-envelope calculation.

Each of the tick box will be marked once the crosscheck is obtained.
After all of them are marked we will close the issue, and be sure that the EC computation is reliable.

[cosimoNigro]

As for the crosscheck against the literature this is done in the tutorials.
https://agnpy.readthedocs.io/en/latest/tutorials/external_compton.html
@jsitarek though pointed out that he cannot reproduce results in some old MAGIC publications
https://github.com/cosimoNigro/agnpy/blob/master/tests/crosschecks/ec_js.ipynb
we have to check this before marking the tick box.

[cosimoNigro]

crosscheck against back-of-the-envelope calculation where started by @jsitarek in
https://github.com/cosimoNigro/agnpy/blob/master/tests/basic/simple_ec.ipynb
and commented in Issues #12 and #13.

[cosimoNigro]

I think the introduction of the test suite addresses the pending checks in this issue.
We have now the following automatic crosschecks for the external Compton radiation:
- crosscheck for EC on Disk against the literature (Finke 2016)

- crosscheck for EC on Broad Line Region against the literature (Finke 2016)

- crosscheck for EC on Dust Torus against the literature (Finke 2016)

As for the crosschecks against back-of-the-envelope calculation I have implemented two test functions that check, in the limit of large distances, if the EC on the BLR (or DT) tends to the one of a point-like source approximating them.

The crosschecks against another code are a bit difficult since Jetset uses different geometries for the targets.
Nonetheless I am confident the External Compton calculations in agnpy are reliable.