Use gammapy in core library

[maxnoe]

Now with the decision for gammapy and gammapy 0.19, I think we can start relying more on gammapy itself.

This will solve several issues

• Avoid duplicated code like spectral models and io
• Better data structures than just a bunch of quantities / numpy arrays by directly using gammapy irf classes

Specifically, this should – once completed – fix issues #151 #48 #21 #31

[codecov]

Codecov Report

Merging #170 (6565f0c) into master (934f2da) will decrease coverage by 0.29%.
The diff coverage is 98.00%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
- Coverage   91.10%   90.80%   -0.30%     
==========================================
  Files          41       41              
  Lines        1607     1566      -41     
==========================================
- Hits         1464     1422      -42     
- Misses        143      144       +1     

Impacted Files	Coverage Δ
pyirf/gammapy.py	92.85% <ø> (ø)
pyirf/irf/effective_area.py	95.65% <90.00%> (-4.35%)	⬇️
pyirf/irf/energy_dispersion.py	100.00% <100.00%> (ø)
pyirf/irf/psf.py	100.00% <100.00%> (ø)
pyirf/irf/tests/test_effective_area.py	100.00% <100.00%> (ø)
pyirf/irf/tests/test_energy_dispersion.py	100.00% <100.00%> (ø)
pyirf/irf/tests/test_psf.py	100.00% <100.00%> (ø)

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