EC on CMB comparison, make uniform the SED format

[cosimoNigro]

In this PR I have:

1. added a script to generate a SED for EC on CMB with jetset, to be compared against the same calculation by agnpy;
2. added this comparison to the automatic tests;
3. worked to standardise our MWL SED data format, they comply now with the GADF specification and can be directly read via Gammapy's FluxPoints.

[codecov[bot]]

Codecov Report

Merging #102 (6e474b7) into master (ad70854) will increase coverage by 0.64%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   95.68%   96.33%   +0.64%     
==========================================
  Files          30       30              
  Lines        2200     2208       +8     
==========================================
+ Hits         2105     2127      +22     
+ Misses         95       81      -14     

Flag	Coverage Δ
unittests	96.33% <100.00%> (+0.64%)	:arrow_up:

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Impacted Files	Coverage Δ
agnpy/tests/test_synchrotron.py	100.00% <ø> (ø)
agnpy/tests/test_compton.py	100.00% <100.00%> (ø)
agnpy/compton/external_compton.py	98.43% <0.00%> (+10.93%)	:arrow_up:

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[sourcery-ai[bot]]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 0.49%.

Quality metrics	Before	After	Change
Complexity	0.10 ⭐	0.10 ⭐	0.00
Method Length	110.20 🙂	108.33 🙂	-1.87 👍
Working memory	9.36 🙂	9.28 🙂	-0.08 👍
Quality	65.77% 🙂	66.26% 🙂	0.49% 👍

Other metrics	Before	After	Change
Lines	658	691	33

Changed files	Quality Before	Quality After	Quality Change
agnpy/data/reference_seds/jetset/jetset_ssa_seds.py	52.24% 🙂	52.24% 🙂	0.00%
agnpy/data/reference_seds/jetset/jetset_ssc_seds.py	57.98% 🙂	57.98% 🙂	0.00%
agnpy/tests/test_compton.py	71.01% 🙂	71.67% 🙂	0.66% 👍
agnpy/tests/test_synchrotron.py	69.40% 🙂	69.40% 🙂	0.00%

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
agnpy/tests/test_synchrotron.py	TestSynchrotron.test_ssa_reference_sed	0 ⭐	172 😞	14 😞	54.13% 🙂	Try splitting into smaller methods. Extract out complex expressions

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[cosimoNigro]

I have also updated the tutorial notebooks to work with the new data and with some of the adjustments we made for the paper. I will add also the gammapy's notebook once we finalise the paper content.

[cosimoNigro]

Merging without review since the core library is not affected, only the test and docs.