Add TrendTransform

[alex-hse-repository]

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

Add TrendTransform

Related Issue

Closing issues

[codecov-commenter]

Codecov Report

Merging #139 (e560206) into master (b11f399) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
+ Coverage   86.97%   87.13%   +0.16%     
==========================================
  Files          70       71       +1     
  Lines        3055     3094      +39     
==========================================
+ Hits         2657     2696      +39     
  Misses        398      398              

Impacted Files	Coverage Δ
etna/transforms/__init__.py	100.00% <100.00%> (ø)
etna/transforms/trend.py	100.00% <100.00%> (ø)

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