Fix/simple statistics

[gnwhr]

Fixes #719 .

Summary

Replaced "Simple statistics" methods with new functions that accept stochastic TimeSeries.

[codecov-commenter]

Codecov Report

Merging #773 (a3c6507) into master (ec6ccf8) will decrease coverage by 0.51%.
The diff coverage is 20.37%.

@@            Coverage Diff             @@
##           master     #773      +/-   ##
==========================================
- Coverage   90.80%   90.29%   -0.52%     
==========================================
  Files          67       68       +1     
  Lines        6752     6777      +25     
==========================================
- Hits         6131     6119      -12     
- Misses        621      658      +37     

Impacted Files	Coverage Δ
darts/metrics/metrics.py	100.00% <ø> (ø)
darts/timeseries.py	82.87% <18.86%> (-3.16%)	⬇️
darts/models/forecasting/theta.py	89.14% <100.00%> (ø)
darts/utils/data/inference_dataset.py	94.54% <0.00%> (-1.82%)	⬇️
darts/models/forecasting/regression_model.py	96.61% <0.00%> (-1.14%)	⬇️
darts/models/forecasting/forecasting_model.py	96.55% <0.00%> (-0.82%)	⬇️
darts/utils/timeseries_generation.py	95.49% <0.00%> (ø)
darts/models/forecasting/random_forest.py	100.00% <0.00%> (ø)
darts/models/forecasting/gradient_boosted_model.py	100.00% <0.00%> (ø)
...arts/models/forecasting/linear_regression_model.py	100.00% <0.00%> (ø)
... and 4 more

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[hrzn]

This looks pretty good, thanks! I added a few comments.
Could you also add some unit tests? Thanks :)

[hrzn]

Looks good! 👍

[hrzn]

👍