fix statistics

[iKintosh]

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Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

Related Issue

Closing issues

[codecov-commenter]

Codecov Report

Merging #523 (d48231b) into master (d80a910) will decrease coverage by 34.05%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master     #523       +/-   ##
===========================================
- Coverage   87.99%   53.93%   -34.06%     
===========================================
  Files         116      116               
  Lines        5538     5538               
===========================================
- Hits         4873     2987     -1886     
- Misses        665     2551     +1886     

Impacted Files	Coverage Δ
etna/transforms/math/statistics.py	64.96% <0.00%> (-33.58%)	⬇️
etna/commands/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
etna/commands/backtest_command.py	0.00% <0.00%> (-96.43%)	⬇️
etna/commands/forecast_command.py	0.00% <0.00%> (-92.00%)	⬇️
etna/commands/__main__.py	0.00% <0.00%> (-87.50%)	⬇️
etna/commands/resolvers.py	0.00% <0.00%> (-80.00%)	⬇️
etna/analysis/outliers/density_outliers.py	22.44% <0.00%> (-75.52%)	⬇️
etna/transforms/timestamp/time_flags.py	26.02% <0.00%> (-73.98%)	⬇️
etna/datasets/datasets_generation.py	26.47% <0.00%> (-73.53%)	⬇️
etna/transforms/timestamp/fourier.py	27.08% <0.00%> (-72.92%)	⬇️
... and 67 more

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