Feat/kalman forecaster

[hrzn]

Fixes: #661 #735

• Introduces KalmanForecaster, a new forecasting model based on KalmanFilter
• Improved KalmanFilter to handle missing values
• Improved KalmanFilter to return a series with columns names matching that of the original series
• Improved ForecastingModels to return predicted series with a time axis having the same name as the original series being forecasted
• Added new unit tests for KalmanForecaster
• Other small improvements.

[codecov-commenter]

Codecov Report

Merging #743 (3f224d6) into master (0e5b5ad) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #743      +/-   ##
==========================================
+ Coverage   90.67%   90.72%   +0.04%     
==========================================
  Files          67       68       +1     
  Lines        6711     6746      +35     
==========================================
+ Hits         6085     6120      +35     
  Misses        626      626              

Impacted Files	Coverage Δ
darts/models/__init__.py	76.19% <100.00%> (+0.58%)	⬆️
darts/models/filtering/kalman_filter.py	98.76% <100.00%> (+0.04%)	⬆️
darts/models/forecasting/forecasting_model.py	96.55% <100.00%> (ø)
darts/models/forecasting/kalman_forecaster.py	100.00% <100.00%> (ø)
darts/utils/timeseries_generation.py	95.49% <100.00%> (ø)

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[brunnedu]

Looks good to me, but @tomasvanpottelbergh could probably give better feedback 😄

[hrzn]

Examples

Forecasting:

Missing values imputation on a multivariate series:

[tomasvanpottelbergh]

Great improvement and addition, thanks! :) LGTM!