Add FourierTransform

[Mr-Geekman]

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

See #292. But there was some change in constructors and mods indexation.

Related Issue

#292.

Closing issues

Closes #292.

[codecov-commenter]

Codecov Report

Merging #306 (ee94bb1) into master (38623dc) will increase coverage by 0.83%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
+ Coverage   87.16%   88.00%   +0.83%     
==========================================
  Files          97       98       +1     
  Lines        4762     4811      +49     
==========================================
+ Hits         4151     4234      +83     
+ Misses        611      577      -34     

Impacted Files	Coverage Δ
etna/transforms/__init__.py	100.00% <100.00%> (ø)
etna/transforms/fourier.py	100.00% <100.00%> (ø)
etna/loggers/base.py	94.23% <0.00%> (+1.92%)	⬆️
etna/transforms/pytorch_forecasting.py	95.29% <0.00%> (+7.05%)	⬆️
etna/core/mixins.py	94.73% <0.00%> (+15.78%)	⬆️
etna/models/nn/tft.py	100.00% <0.00%> (+22.64%)	⬆️
etna/models/nn/deepar.py	100.00% <0.00%> (+23.52%)	⬆️

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[iKintosh]

Other than the comment it looks great!

[iKintosh]

Thanks 👍