add _filter to variance and mean

[StRigaud]

closes #380

@haesleinhuepf _box and _phere version still exist.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 47.70%. Comparing base (4e8662d) to head (4ed614e).

Files with missing lines	Patch %	Lines
clic/src/tier2/detect_maxima.cpp	0.00%	0 Missing and 1 partial ⚠️
clic/src/tier2/detect_maxima_box.cpp	0.00%	1 Missing ⚠️
clic/src/tier2/detect_minima.cpp	0.00%	0 Missing and 1 partial ⚠️
clic/src/tier2/detect_minima_box.cpp	0.00%	1 Missing ⚠️
clic/src/tier2/standard_deviation.cpp	0.00%	1 Missing ⚠️
clic/src/tier2/standard_deviation_box.cpp	0.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #381   +/-   ##
=======================================
  Coverage   47.70%   47.70%           
=======================================
  Files         243      243           
  Lines        4000     4000           
  Branches     1830     1830           
=======================================
  Hits         1908     1908           
  Misses        660      660           
  Partials     1432     1432           

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