Tbas adapter

[scanhex12]

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 #678

[github-actions]

🚀 Deployed on https://deploy-preview-734--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #734 (4879636) into master (f9fb07c) will increase coverage by 0.12%.
The diff coverage is 95.55%.

@@            Coverage Diff             @@
##           master     #734      +/-   ##
==========================================
+ Coverage   83.58%   83.71%   +0.12%     
==========================================
  Files         121      122       +1     
  Lines        6654     6699      +45     
==========================================
+ Hits         5562     5608      +46     
+ Misses       1092     1091       -1     

Impacted Files	Coverage Δ
etna/models/tbats.py	95.34% <95.34%> (ø)
etna/models/__init__.py	100.00% <100.00%> (ø)
etna/core/mixins.py	95.65% <0.00%> (+13.04%)	⬆️

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[julia-shenshina]

Looks great ⭐

Some small fixes:

• you should update CHANGELOG.md (section Unreleased -> Added)
• fill the header of the PR please:
  • choose task type
  • add issue to closes issue section - it is important to close the corresponding issue when merge the PR

[julia-shenshina]

Looks great! 🔥