Fix order of columns after to flatten

[GooseIt]

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

It may be useful to impose the same order on the return dataframe of .to_dataset() for the sake of consistency. Current order of columns in the return dataframe of ._to_dataset() places "target" along other features in alphabetical order.

Closing issues

closes #873

[GooseIt]

The "Proposed Changes" paragraph may be misleading.
The pull requests solves all problems and only those problems outlined in issue#873.
I wrote "Proposed Changes" as a statement of what can be further done in the direction established by issue#873 after the issue is closed. If I was wrong in doing this, please inform me.

[codecov-commenter]

Codecov Report

Merging #1095 (71ddc29) into master (d65999e) will decrease coverage by 18.53%.
The diff coverage is 100.00%.

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@@             Coverage Diff             @@
##           master    #1095       +/-   ##
===========================================
- Coverage   86.96%   68.44%   -18.53%     
===========================================
  Files         170      170               
  Lines        9324     9321        -3     
===========================================
- Hits         8109     6380     -1729     
- Misses       1215     2941     +1726     

Impacted Files	Coverage Δ
etna/datasets/tsdataset.py	74.94% <100.00%> (-16.96%)	⬇️
etna/commands/__main__.py	0.00% <0.00%> (-87.50%)	⬇️
etna/datasets/hierarchical_structure.py	22.01% <0.00%> (-75.23%)	⬇️
etna/libs/pmdarima_utils/arima.py	28.84% <0.00%> (-71.16%)	⬇️
etna/pipeline/assembling_pipelines.py	26.31% <0.00%> (-71.06%)	⬇️
etna/pipeline/mixins.py	31.42% <0.00%> (-67.62%)	⬇️
etna/analysis/outliers/hist_outliers.py	10.82% <0.00%> (-66.25%)	⬇️
etna/reconciliation/bottom_up.py	35.29% <0.00%> (-64.71%)	⬇️
etna/auto/optuna/config_sampler.py	32.25% <0.00%> (-62.91%)	⬇️
etna/models/deadline_ma.py	35.18% <0.00%> (-59.26%)	⬇️
... and 59 more

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[Mr-Geekman]

Could you please run make format command to fix linting ci stage?

[Mr-Geekman]

Proposed Changes section is about what is done inside PR, not about what we want to do in the future. I agree that it can be unclear, I usually just leave a link to the issue in this section.

[GooseIt]

Fixed issues with codestyle and ran 'make format' command