Add get_anomalies_iqr function

[brsnw250]

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?

Proposed Changes

Closing issues

closes #346

[github-actions]

🚀 Deployed on https://deploy-preview-374--etna-docs.netlify.app

[d-a-bunin]

Can't we reference it using :py:class syntax?

[codecov]

Codecov Report

Attention: Patch coverage is 93.18182% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.81%. Comparing base (5cb8485) to head (24402c0).
Report is 5 commits behind head on master.

Files	Patch %	Lines
etna/analysis/outliers/rolling_statistics.py	93.02%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
- Coverage   88.88%   86.81%   -2.08%     
==========================================
  Files         224      226       +2     
  Lines       15381    15531     +150     
==========================================
- Hits        13672    13483     -189     
- Misses       1709     2048     +339     

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