Create DirectEnsemble

[alex-hse-repository]

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 #766

[github-actions]

🚀 Deployed on https://deploy-preview-824--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #824 (ab90055) into master (5efc8f5) will decrease coverage by 34.95%.
The diff coverage is 40.42%.

@@             Coverage Diff             @@
##           master     #824       +/-   ##
===========================================
- Coverage   84.34%   49.38%   -34.96%     
===========================================
  Files         126      127        +1     
  Lines        7238     7285       +47     
===========================================
- Hits         6105     3598     -2507     
- Misses       1133     3687     +2554     

Impacted Files	Coverage Δ
etna/ensembles/direct_ensemble.py	39.13% <39.13%> (ø)
etna/ensembles/__init__.py	100.00% <100.00%> (ø)
etna/commands/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
etna/commands/backtest_command.py	0.00% <0.00%> (-97.06%)	⬇️
etna/commands/forecast_command.py	0.00% <0.00%> (-94.88%)	⬇️
etna/commands/__main__.py	0.00% <0.00%> (-87.50%)	⬇️
etna/models/utils.py	14.28% <0.00%> (-85.72%)	⬇️
etna/commands/resolvers.py	0.00% <0.00%> (-80.00%)	⬇️
etna/analysis/outliers/density_outliers.py	22.44% <0.00%> (-75.52%)	⬇️
etna/datasets/datasets_generation.py	27.02% <0.00%> (-72.98%)	⬇️
... and 78 more

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

[Mr-Geekman]

Fix typos and check questions.