Feat/improve seasonal decompose

[gnwhr]

Fixes #832

Summary

Added more robust STD decomposition method as an alternative to the naive method.

[codecov-commenter]

Codecov Report

Merging #862 (c34bc5f) into master (a7abedf) will increase coverage by 0.16%.
The diff coverage is 97.50%.

@@            Coverage Diff             @@
##           master     #862      +/-   ##
==========================================
+ Coverage   91.40%   91.57%   +0.16%     
==========================================
  Files          70       71       +1     
  Lines        7106     7154      +48     
==========================================
+ Hits         6495     6551      +56     
+ Misses        611      603       -8     

Impacted Files	Coverage Δ
darts/utils/statistics.py	52.65% <90.90%> (+6.25%)	⬆️
darts/utils/losses.py	100.00% <100.00%> (ø)
...arts/models/forecasting/linear_regression_model.py	94.73% <0.00%> (-0.58%)	⬇️
darts/models/forecasting/gradient_boosted_model.py	100.00% <0.00%> (ø)
darts/models/forecasting/regression_model.py	97.11% <0.00%> (+0.34%)	⬆️

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[hrzn]

Looks good, just wondering whether we should raise an error instead of warning when model is not additive.

[hrzn]

shouldn't we raise an error here?

[gnwhr]

Yes, would be better. I will change it.