Add sum for omegaconf resolvers

[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

[codecov-commenter]

Codecov Report

Merging #300 (abc8d97) into master (4b1a41c) will increase coverage by 0.91%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage   86.72%   87.63%   +0.91%     
==========================================
  Files          92       95       +3     
  Lines        4489     4683     +194     
==========================================
+ Hits         3893     4104     +211     
+ Misses        596      579      -17     

Impacted Files	Coverage Δ
etna/analysis/__init__.py	100.00% <100.00%> (ø)
etna/analysis/outliers/__init__.py	100.00% <100.00%> (ø)
.../analysis/outliers/prediction_interval_outliers.py	90.90% <100.00%> (ø)
etna/commands/__init__.py	100.00% <100.00%> (ø)
etna/ensembles/stacking_ensemble.py	99.11% <100.00%> (ø)
etna/ensembles/voting_ensemble.py	100.00% <100.00%> (ø)
etna/models/base.py	97.50% <100.00%> (+0.03%)	⬆️
etna/models/prophet.py	98.29% <100.00%> (+0.02%)	⬆️
etna/models/sarimax.py	95.94% <100.00%> (+0.29%)	⬆️
etna/pipeline/autoregressive_pipeline.py	100.00% <100.00%> (ø)
... and 13 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 46a8825...abc8d97. Read the comment docs.

[martins0n]

👍