Fix Li & Ma significance for n_off = 0

[maxnoe]

This adds a check to give the correct limit of the li & ma formula for n_off -> 0, also adds test to make sure we get the same values as gammapy

[codecov]

Codecov Report

Merging #86 into master will increase coverage by 1.62%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   83.05%   84.68%   +1.62%     
==========================================
  Files          33       34       +1     
  Lines        1021     1038      +17     
==========================================
+ Hits          848      879      +31     
+ Misses        173      159      -14     

Impacted Files	Coverage Δ
pyirf/statistics.py	94.44% <100.00%> (+77.77%)	⬆️
pyirf/tests/test_statistics.py	100.00% <100.00%> (ø)

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