Use vectorize for relative_sensitivity

[maxnoe]

Fixes #105

[codecov]

Codecov Report

Merging #106 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   89.18%   89.25%   +0.07%     
==========================================
  Files          36       36              
  Lines        1193     1201       +8     
==========================================
+ Hits         1064     1072       +8     
  Misses        129      129              

Impacted Files	Coverage Δ
pyirf/sensitivity.py	92.06% <100.00%> (+0.53%)	⬆️
pyirf/tests/test_sensitivity.py	100.00% <100.00%> (ø)
pyirf/version.py	100.00% <100.00%> (ø)

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