Add cap and floor to prophet regressor columns

[Mr-Geekman]

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Related Issue

#58.

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Closes #58.

[codecov-commenter]

Codecov Report

Merging #62 (b6f3ea3) into master (ada6523) will decrease coverage by 4.69%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   90.49%   85.80%   -4.70%     
==========================================
  Files          45       45              
  Lines        2010     2015       +5     
==========================================
- Hits         1819     1729      -90     
- Misses        191      286      +95     

Impacted Files	Coverage Δ
etna/models/prophet.py	98.70% <100.00%> (+0.10%)	⬆️
etna/transforms/pytorch_forecasting.py	26.38% <0.00%> (-73.62%)	⬇️
etna/models/nn/tft.py	38.88% <0.00%> (-61.12%)	⬇️
etna/models/nn/deepar.py	41.17% <0.00%> (-58.83%)	⬇️
etna/metrics/base.py	96.29% <0.00%> (-0.07%)	⬇️

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[iKintosh]

🔥

[julia-shenshina]

elif column_name in ["cap", "floor"]:
   regressors_columns.append(column_name)

❓

[julia-shenshina]

shouldn't it be regressor_cap and regressor_floor according to our columns naming rules?