Add BinsegTrendTransform, ChangePointsTrendTransform

[julia-shenshina]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

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

closes #69

[codecov-commenter]

Codecov Report

Merging #87 (99a99c9) into master (d48e4f5) will increase coverage by 0.58%.
The diff coverage is 99.23%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   88.04%   88.63%   +0.58%     
==========================================
  Files          50       52       +2     
  Lines        2283     2410     +127     
==========================================
+ Hits         2010     2136     +126     
- Misses        273      274       +1     

Impacted Files	Coverage Δ
etna/transforms/binseg.py	96.96% <96.96%> (ø)
etna/transforms/__init__.py	100.00% <100.00%> (ø)
etna/transforms/change_points_trend.py	100.00% <100.00%> (ø)
etna/transforms/detrend.py	98.21% <0.00%> (-0.07%)	⬇️
etna/transforms/datetime_flags.py	100.00% <0.00%> (ø)

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[martins0n]

🔥
Tests are awesome!