Verify hdus using ogadf-schema

[maxnoe]

This adds tests verifying that the hdus created by pyirf.io.gadf are schema compliant to the GADF schemas using ogadf-schema

[codecov]

Codecov Report

Merging #63 into master will increase coverage by 2.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   74.03%   76.27%   +2.24%     
==========================================
  Files          31       31              
  Lines         751      784      +33     
==========================================
+ Hits          556      598      +42     
+ Misses        195      186       -9     

Impacted Files	Coverage Δ
pyirf/io/tests/test_gadf.py	98.94% <100.00%> (+0.56%)	⬆️
pyirf/io/gadf.py	98.68% <0.00%> (+11.84%)	⬆️

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[vuillaut]

This looks good - shall we merge?

[maxnoe]

yes