Clean up gammapy.stats

[registerrier]

Description

This pull request is a clean-up of gammapy.stats. It removes poisson.py and normal.py.

To do so an excess_matching_significance method is added to the CountsStatistic base class.
The significance and significance_on_off calls in the code are replace with CashCountsStatistic.significance and WStatCountsStatistic.significance respectively.

The helene_ul function is removed as well as the significance_to_probability functions which are never used in the code base.

Dear reviewer

[codecov]

Codecov Report

Merging #2842 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2842      +/-   ##
==========================================
- Coverage   92.18%   92.18%   -0.01%     
==========================================
  Files         147      145       -2     
  Lines       16771    16588     -183     
==========================================
- Hits        15461    15292     -169     
+ Misses       1310     1296      -14     

Impacted Files	Coverage Δ
gammapy/stats/__init__.py	100.00% <ø> (ø)
gammapy/datasets/spectrum.py	95.39% <100.00%> (ø)
gammapy/estimators/asmooth_map.py	92.70% <100.00%> (ø)
gammapy/estimators/sensitivity.py	100.00% <100.00%> (ø)
gammapy/stats/counts_statistic.py	100.00% <100.00%> (ø)

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[adonath]

Thanks @registerrier, so far I have no comments, I'll go ahead an merge and polish in master if needed...