Add shorter tags for models

[QRemy]

Redefine most of models tags with a list including shorter aliases to be used with model.create(tag) and in YAML serialization. By default the tag returned in model.to_dict() is the 0th tag which is also the class name.

[codecov]

Codecov Report

Merging #2938 into master will decrease coverage by 0.03%.
The diff coverage is 99.09%.

@@            Coverage Diff             @@
##           master    #2938      +/-   ##
==========================================
- Coverage   93.38%   93.34%   -0.04%     
==========================================
  Files         147      147              
  Lines       16957    16991      +34     
==========================================
+ Hits        15835    15860      +25     
- Misses       1122     1131       +9     

Impacted Files	Coverage Δ
gammapy/makers/safe.py	93.90% <83.33%> (+1.13%)	⬆️
gammapy/catalog/fermi.py	95.41% <100.00%> (ø)
gammapy/catalog/gammacat.py	90.64% <100.00%> (ø)
gammapy/catalog/hawc.py	98.79% <100.00%> (ø)
gammapy/catalog/hess.py	97.21% <100.00%> (ø)
gammapy/datasets/spectrum.py	96.45% <100.00%> (+0.08%)	⬆️
gammapy/estimators/sensitivity.py	100.00% <100.00%> (ø)
gammapy/irf/edisp_map.py	94.28% <100.00%> (+0.34%)	⬆️
gammapy/makers/spectrum.py	98.50% <100.00%> (+0.04%)	⬆️
gammapy/maps/region.py	89.90% <100.00%> (+0.40%)	⬆️
... and 9 more

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[adonath]

Thanks a lot @QRemy! As discussed in the meeting this morning, please adapt the YAML tags to the one proposed in PIG 21 here: https://github.com/gammapy/gammapy/blob/0ab86da3d9fca7046e67b40f23332442d1e715b9/docs/development/pigs/pig-21.rst#simplify-yaml-representation

I'll continue with the review this afternoon...

[adonath]

Thanks a lot @QRemy, I have no further comments. We just should add some documentation on this soon...