Store correct signal amount, store information on which checks failed for sensitivity bins

[maxnoe]

This just adds a column on which of the 3 checks failed to the output

[codecov]

Codecov Report

Merging #97 into master will increase coverage by 2.64%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
+ Coverage   87.08%   89.72%   +2.64%     
==========================================
  Files          35       36       +1     
  Lines        1138     1256     +118     
==========================================
+ Hits          991     1127     +136     
+ Misses        147      129      -18     

Impacted Files	Coverage Δ
pyirf/sensitivity.py	91.52% <100.00%> (+24.28%)	⬆️
pyirf/tests/test_sensitivity.py	100.00% <100.00%> (ø)
pyirf/tests/test_eventdisplay_comparison.py	100.00% <0.00%> (ø)
pyirf/io/tests/test_gadf.py	99.16% <0.00%> (+0.23%)	⬆️
pyirf/io/gadf.py	100.00% <0.00%> (+1.31%)	⬆️
pyirf/utils.py	85.29% <0.00%> (+8.82%)	⬆️

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