Prediction intervals visualization

[Ama16]

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• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
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Related Issue

Closing issues

closes #267

[Ama16]

[codecov-commenter]

Codecov Report

Merging #538 (5a6d266) into master (a7a08c8) will decrease coverage by 1.06%.
The diff coverage is 7.40%.

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
- Coverage   88.21%   87.15%   -1.07%     
==========================================
  Files         118      118              
  Lines        5677     5767      +90     
==========================================
+ Hits         5008     5026      +18     
- Misses        669      741      +72     

Impacted Files	Coverage Δ
etna/analysis/plotters.py	10.61% <7.40%> (-1.25%)	⬇️
etna/models/base.py	77.44% <0.00%> (-16.38%)	⬇️
etna/models/nn/tft.py	100.00% <0.00%> (ø)
etna/models/nn/deepar.py	100.00% <0.00%> (ø)
etna/analysis/__init__.py	100.00% <0.00%> (ø)

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[Ama16]

[martins0n]

Seems great!