HierarchicalStructure class

[brsnw250]

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

An implementation of the class that holds and manages a hierarchical structure.

Closing issues

closes #1041

[codecov-commenter]

Codecov Report

Merging #1044 (50e764c) into hierarchical_pipeline (b8e45f8) will decrease coverage by 16.27%.
The diff coverage is 22.85%.

@@                    Coverage Diff                     @@
##           hierarchical_pipeline    #1044       +/-   ##
==========================================================
- Coverage                  85.99%   69.72%   -16.28%     
==========================================================
  Files                        162      164        +2     
  Lines                       8628     8729      +101     
==========================================================
- Hits                        7420     6086     -1334     
- Misses                      1208     2643     +1435     

Impacted Files	Coverage Δ
etna/datasets/hierarchical_structure.py	22.11% <22.11%> (ø)
etna/datasets/__init__.py	100.00% <100.00%> (ø)
etna/commands/__main__.py	0.00% <0.00%> (-87.50%)	⬇️
etna/libs/pmdarima_utils/arima.py	28.84% <0.00%> (-71.16%)	⬇️
etna/pipeline/assembling_pipelines.py	26.31% <0.00%> (-71.06%)	⬇️
etna/analysis/outliers/hist_outliers.py	10.82% <0.00%> (-66.25%)	⬇️
etna/auto/optuna/config_sampler.py	32.25% <0.00%> (-62.91%)	⬇️
etna/models/deadline_ma.py	35.18% <0.00%> (-59.26%)	⬇️
etna/pipeline/mixins.py	38.77% <0.00%> (-59.19%)	⬇️
etna/clustering/distances/distance_matrix.py	32.83% <0.00%> (-58.21%)	⬇️
... and 56 more

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