ADD: greater_is_better property

[martins0n]

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

Closing issues

[github-actions]

🚀 Deployed on https://deploy-preview-921--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #921 (4a36eb6) into master (7bff536) will decrease coverage by 0.26%.
The diff coverage is 71.73%.

@@            Coverage Diff             @@
##           master     #921      +/-   ##
==========================================
- Coverage   85.04%   84.77%   -0.27%     
==========================================
  Files         133      133              
  Lines        7548     7593      +45     
==========================================
+ Hits         6419     6437      +18     
- Misses       1129     1156      +27     

Impacted Files	Coverage Δ
etna/metrics/intervals_metrics.py	94.44% <66.66%> (-3.48%)	⬇️
etna/metrics/metrics.py	86.44% <66.66%> (-13.56%)	⬇️
etna/metrics/base.py	93.67% <81.25%> (-3.21%)	⬇️
etna/libs/tsfresh/significance_tests.py	31.88% <0.00%> (-14.50%)	⬇️
etna/transforms/nn/pytorch_forecasting.py	90.47% <0.00%> (-4.77%)	⬇️

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